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.