minor typo fix on Manage Jenkins page

Added a trailing period to english AboutJenkins.Description because it
was the only message on that page having none.

core/src/main/resources/hudson/Messages.properties

@@ -62,4 +62,4 @@ PluginManager.ConfigureUpdateCenterPermission.Description=\
 configure update sites and proxy settings.
 
 AboutJenkins.DisplayName=About Jenkins
-AboutJenkins.Description=See the version and license information
+AboutJenkins.Description=See the version and license information.