Update Mathlib/Combinatorics/Enumerative/Pentagonal.lean

Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>

Author: wwylele

Mathlib/Combinatorics/Enumerative/Pentagonal.lean

@@ -12,7 +12,7 @@ public import Mathlib.Data.Int.SuccPred
 
 This file introduces (generalized) pentagonal numbers $k(3k-1)/2$ for integer $k$.
 
-Some source, such as A001318 in the OEIS, orders generalized pentagonal numbers by indices
+Some sources, such as A001318 in the OEIS, order generalized pentagonal numbers by indices
 $k = 0, 1, -1, 2, -2, \cdots$ to form a strictly monotone sequence. This file doesn't follow this
 convention, but implicitly shows the monotonicity by `pentagonal_lt_pentagonal_neg` and
 `pentagonal_neg_lt_pentagonal_add_one`.