incorrect variable in bp_activity_notifications

[peterspeidel]

see line 18 and 19

		// Now email the user with the contents of the message (if they have enabled email notifications)
		if ( 'no' != get_usermeta( $user_id, 'notification_activity_new_mention' ) ) 

this should be standing there

		// Now email the user with the contents of the message (if they have enabled email notifications)
		if ( 'no' != get_usermeta( $receiver_user_id, 'notification_activity_new_mention' ) )

in other words: replace $user_id for $receiver_user_id

[apeatling]

(In [2869]) Fixes #2235 props peterspeidel