Clarify errhint in sources.sgml

It was noted in 20230712015948.byqaftt57glwknjz@awork3.anarazel.de that the
errhint example in sources.sgml isn't as helpful as it could be.  errhint
should use a complete sentence, but the example doesn't, so I propose the
attached change which makes it so.  The style for hints is clearly spelled out
further down on the page, but for anyone looking for a quick answer and not
reading the whole page the current example might be misleading.

--
Daniel Gustafsson