doc: fix a couple of typos

[Nathan Sidwell]

gty calls gt_clear*e*_cache not gt_clear_cache.  I know not why it is 
named so, but at least document it correctly.  invoke.texi had a 
duplicate opindex.

             gcc/
             * doc/gty.texi: Fic gt_cleare_cache name.
             * doc/invoke.texi: Remove duplicate opindex Wabi-tag.

pushing to trunk
-- 
Nathan Sidwell
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