Fix typo in comment

Reported-by: Oleg Bartunov

src/backend/utils/adt/tsrank.c

@@ -336,7 +336,7 @@ calc_rank_or(const float *w, TSVector t, TSQuery q)
 				}
 			}
 /*
-			limit (sum(i/i^2),i->inf) = pi^2/6
+			limit (sum(1/i^2),i=1,inf) = pi^2/6
 			resj = sum(wi/i^2),i=1,noccurence,
 			wi - should be sorted desc,
 			don't sort for now, just choose maximum weight. This should be corrected