Fix operator naming in pg_trgm GUC option descriptions

Descriptions of pg_trgm GUC options have % replaced with %% like it was
a printf-like format.  But that's not needed since they are just plain strings.
This commit fixed that.  Backpatch to last supported version since this error
present from the beginning.

Reported-by: Masahiko Sawada
Discussion: https://postgr.es/m/CAD21AoAgPKODUsu9gqUFiNqEOAqedStxJ-a0sapsJXWWAVp%3Dxg%40mail.gmail.com
Backpatch-through: 9.4