PEP 658: Fix attribute description in abstract

[uranusjr]

During the PEP's evoltion, the data-dist-info-metadata attribute was changed from indicating the metadata's location to merely indicating the file's existence. The exact location should be inferred elsewhere. But we forgot to update the description in the abstract to reflect this.

This was mentioned by @dstufft in the resolution message:

I will mention there’s one bit of wording that you might want to clean up. It doesn’t really affect the contents of the PEP, but the Abstract still references data-dist-info-metadata as the pointer for the location of the metadata file.