A page number can change based on the edition you are using, but the quote appears in part I, chapter 4 of the text. In my Signet edition, it is on page 36 and reads as follows:

As short a time ago as February, the Ministry of Plenty had issued a promise (a "categorical pledge" were the official words) that there would be no reduction of the chocolate ration during 1984. Actually, as Winston was aware, the chocolate ration was to be reduced from thirty grammes to twenty at the end of the present week. All that was needed was to substitute for the original promise a warning that it would probably be necessary to reduce the ration at some time in April.

Winston is one of the people whose job it is to make the changes in the historical record that make it appear as if the government never lies or contradicts itself. He will go back and change the government pledge not to reduce the chocolate ration so that it matches the current reality of a reduction in the chocolate ration. By having people like Winston constantly rewriting history, the Party ensures that truth becomes hopelessly obscured. It then becomes ever easier for the Party to assert that the truth is whatever the Party claims it to be. Meanwhile, people like Winston are expected to engage in "doublethink," which involves both changing the historic record and immediately forgetting that they ever did so. 

However, Winston commits "thought crime": he does not forget. In chapter 5, Winston notes:

It appeared that there had even been demonstrations to thank Big Brother for raising the chocolate ration to twenty grammes a week. And only yesterday, he reflected, it had been announced that the ration was to be REDUCED to twenty grammes a week.

Winston would be acutely aware of all these contradictions as he was the one who just changed facts on the chocolate ration to make the reduction (now posited as an increase) seem consistent with past pronouncements. If you're confused, that's the point: the government in Oceania never wants people to know what is really going on.

In my copy of 1984 (Penguin, London, 1990), you will find this quote on pages 41-42, though it may be slightly different in yours. Either way, this quote appears in Part 1, Chapter 4 of the book, in which Winston is discussing his role at the Ministry of Truth. 

According to Winston, the party made a "categorical pledge" that chocolate rations would not be reduced. This pledge was made in February 1984 but, a few months later, Winston is instructed to rewrite history so as to make this pledge disappear. In its place, Winston writes that the party might have to reduce the ration in April and this paves the way for the news that the ration will, indeed, be reduced: from 30 grammes per person to 20. 

By rewriting the history, the party eradicate all traces of their pledge and make it appear as though it never happened. By suggesting, instead, that the chocolate ration might reduce, the party appear to be honest with the people. In reality, they have simply manipulated the flow of information.