MDL-20438 rewording confusing status_uptodate

The status_uptodate is a flag that the plugin's on-disk version and the
in-db version are in sync, that is no upgrade will be performed.
However, there can be a newer version of the plugin available. So using
the term "up-to-date" may be confusing.

lang/en/plugin.php

@@ -64,7 +64,7 @@ $string['status_missing'] = 'Missing from disk!';
 $string['status_new'] = 'To be installed';
 $string['status_nodb'] = 'No database';
 $string['status_upgrade'] = 'To be upgraded';
-$string['status_uptodate'] = 'Up-to-date';
+$string['status_uptodate'] = 'Installed';
 $string['systemname'] = 'Identifier';
 $string['type_auth'] = 'Authentication method';
 $string['type_auth_plural'] = 'Authentication methods';