doc: Remove index reference to removed documentation in fortran manual

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.