Rephrase pg_conversion description to avoid splitting link across page.

Rephrase pg_conversion description to avoid splitting link across page.

The link to the CREATE CONVERSION manual page was split across a page
boundary in the PDF output, leading to "\pdfendlink ended up in different
nesting level than \pdfstartlink" error while building PDFs.

It wouldn't be worth changing text that's undergoing active editing to
avoid this, since other editing might result in moving the link away from
the page end anyway.  But this paragraph has been static for a long time,
so might as well fix it to prevent it from being an issue in future.