Fix English in wikibase-error-constraint-violation-label

Assuming that documentation is correct, it's not about number or constraints
but of violations to the costraint(s).
Previous wording doesn't make sense in any case.

<https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-error-constraint-violation-label/en>

Change-Id: Idd401798ae0c1a9cb1e8836c48ab24e89deb57f2
Merge "Fix English in wikibase-error-constraint-violation-label"