Fortran option -M used to be an alias for -J. After some deprecation time,
it was reused for another purpose at revision
r0-100725-gd8ddea4044ee8212d5fe305e8e2a547700cd7b8f.
That revision removed the documentation parts of -J mentioning -M, but left
a reference to -M in the index.
This change removes the remaining reference.
gcc/fortran/ChangeLog:
* invoke.texi (-M): Remove index reference to removed documentation.
@option{-I} option.
@opindex J@var{dir}
-@opindex M@var{dir}
@cindex paths, search
@cindex module search path
@item -J@var{dir}