Syntax error: MediaWiki -> Mediawiki

Errors introduced in Id6c2d79d62. When you reference (rather than declare)
Puppet resources, you have to capitalize the first letter of each fragment of
the name. Thus a 'mediawiki::extension' resource would be referred to as
'Mediawiki::Extension'. The 'w' in 'Mediawiki' is not capitalized.

Change-Id: I0942ca16bed252cc654d888d70fa738f9084e3e5