Fix broken model links in v4

[rettinghaus]

This fixes some broken links in the Guidelines.
Closes music-encoding/music-encoding#979

[musicEnfanthen]

Thank you @rettinghaus for the fix. [Hope you don't mind if I changed the title to a more speaking one for future reference.]

@kepper There seemed to be some issue with link generation for model links. We should double check that these links are correctly generated in the upcoming guideline versions.

[musicEnfanthen]

@rettinghaus
There seems to be another one in:

guidelines/v4/content/metadata.html

Line 1269 in 5139d7d

<a href="/guidelines/v4/model-classes/model.model.pubstmtpart.html">model.model.pubStmtPart</a> class. The following elements may be used to provide details regarding the file’s publication and distribution:</p>

[rettinghaus]

Thanks, @musicEnfanthen ! Should be fixed now.

[musicEnfanthen]

Merged at ODD meeting (July 2022). Thanks @rettinghaus for the fix and @apacha for reporting.