Fix broken links in grails-data/gorm documentation

[jamesfredley]

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