minor #16400 Fix bugs seen by _build/build.php diagnostics in 6.0 (MarkPedron)

This PR was merged into the 6.0 branch.

Discussion
----------

Fix bugs seen by `_build/build.php` diagnostics in 6.0

This PR fixes a broken reference found by the build script.

I dealt with the remaining diagnostics
* from 4.4 in #16397.
* from 5.3 in #16398
* from 5.4 in #16399

Commits
-------

96f0174 Add missing anchor in forms/bootstrap5

Author: javiereguiluz

form/bootstrap5.rst

@@ -83,6 +83,8 @@ If you prefer to apply the Bootstrap styles on a form to form basis, include the
     container. If you override the ``row_attr`` class option, the ``mb-3`` will
     be overridden too and you will need to explicitly add it.
 
+.. _reference-forms-bootstrap5-error-messages:
+
 Error Messages
 --------------