Replace "article" with "page" in wikibase-sitelink-page-edit-placeholder

It's not only articles, even the mockup uses "pages":
[[File:Screenshot WikidataRepo 2012-05-13 E.png]]
<https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Wikibase-sitelink-page-edit-placeholder/en>

Followup to aae82d4c

Change-Id: Id4cdb3a9b039a14887913d92d24d0696d85b3ee1
Merge "Replace "article" with "page" in wikibase-sitelink-page-edit-placeholder"