Tutorial: Fix Texinfo dir removal instructions
    
    The instructions gave the impression that the installed Texinfo 'dir'
    file should be done at the very start of %install. Actually, it should
    be done after %make_install is called, so that the file has been copied
    to buildroot.
    
    Fix by clarifying the instructions.
    
    Also, the macros used as parameters to 'rm' expanded to a double slash
    '//'. It worked, but was unclear. Fix by removing literal '/'.