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

contrib/pg_trgm/trgm_op.c

@@ -48,7 +48,7 @@ _PG_init(void)
 {
 	/* Define custom GUC variables. */
 	DefineCustomRealVariable("pg_trgm.similarity_threshold",
-							 "Sets the threshold used by the %% operator.",
+							 "Sets the threshold used by the % operator.",
 							 "Valid range is 0.0 .. 1.0.",
 							 &similarity_threshold,
 							 0.3,
@@ -60,7 +60,7 @@ _PG_init(void)
 							 NULL,
 							 NULL);
 	DefineCustomRealVariable("pg_trgm.word_similarity_threshold",
-							 "Sets the threshold used by the <%% operator.",
+							 "Sets the threshold used by the <% operator.",
 							 "Valid range is 0.0 .. 1.0.",
 							 &word_similarity_threshold,
 							 0.6,