docs: Remove last use of double semicolon in Makefile
    
    Double semicolons have special meaning in makefiles, but they would have
    to be combined with other rules witch such separators in order to be
    used as intended.  Since there are no other rules like that, let's
    clean it up.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>