Fix broken links in grails-data/gorm documentation

### Issue description

Broken links can be identified and verified as fixed using https://github.com/lycheeverse/lychee/releases/.

```
./gradlew grails-doc:build
cd \grails-doc\build\docs\grails-data
lychee-x86_64-windows.exe . --format markdown --output results.md --exclude-path api --exclude-path grails-data/hibernate5/api --exclude ".*.css$" --exclude "^http://localhost"
```

review `results.md`

Build the documentation with ./gradlew grails-doc:build, then run lychee against grails-doc/build/docs/grails-data using the exclusions and output settings in the issue. Review results.md, update the documentation links reported there, and rerun the check until the broken links are fixed.

Written by the indexing model from the issue text.