Fix @opindex for m80387

I noticed that the opindex for -m80387
option was wrong. It was just 80387 which
was not consistent with the rest of the options.
This fixes that and uses "@opindex m80387".

Committed as obvious after  "make html" and checking
the option index page.

gcc/ChangeLog:

	* doc/invoke.texi: Fix @opindex
	for m80387 option.
1 file changed