Add missing period to wikibase-error-serialize-error

For consistency with wikibase-error-invalid-entity-id in same file.
<https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-error-serialize-error/en>

Change-Id: Ia6d76f20a225658bcf0cda3d825221c9c2b3a8df
Merge "Add missing period to wikibase-error-serialize-error"