Update class-metabox.php

Fix documentation typo
Merge git://github.com/jdevalk/wordpress-seo into remove-duplicate-function

Conflicts:
	readme.txt
Merge branch 'remove-duplicate-function' of github.com:jrfnl/wordpress-seo into remove-duplicate-function
Merge branch 'remove-duplicate-function'

Conflicts:
	readme.txt