Removes 'Change 'plugin-name' to the name of your plugin' from DocBlock

Removes 'Change 'plugin-name' to the name of your plugin' from add_plugin_admin_menu() function DocBlock, as this is now defined via a variable.
Merge pull request #79 from barryceelen/patch-2

Removes 'Change 'plugin-name' to the name of your plugin' from DocBlock