[doc, committed] remove leading dash from @opindex entries

[Sandra Loosemore]

I noticed this buglet when working on the PR80828 fix:

The introductory paragraph to the Option Index appendix says: "GCC’s 
command line options are indexed here without any initial ‘-’ or ‘--’." 
Indeed, that was mostly true, but there were ~20 index entries that 
incorrectly included the leading dash.  Fixed thusly.

-Sandra
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