Fix typo in openai.txt

[mirek11s]

There is a typo in openai.txt. The text you provided is in the phrase "also known an as an 'eval'." The corrected version should be "also known as an 'eval'." So, the corrected full text would be:

"Improving performance is easier if you can measure it. In some cases, a modification to a prompt will achieve better performance on a few isolated examples but lead to worse overall performance on a more representative set of examples. Therefore, to be sure that a change is net positive to performance, it may be necessary to define a comprehensive test suite (also known as an 'eval')."