Fix typos / spelling in comments

Signed-off-by: Mike Ralphson <mike@abacus.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/levenshtein.c b/levenshtein.c
index a32f4cd..fc28159 100644
--- a/levenshtein.c
+++ b/levenshtein.c
@@ -27,7 +27,7 @@
  *
  * It does so by calculating the costs of the path ending in characters
  * i (in string1) and j (in string2), respectively, given that the last
- * operation is a substition, a swap, a deletion, or an insertion.
+ * operation is a substitution, a swap, a deletion, or an insertion.
  *
  * This implementation allows the costs to be weighted:
  *