Fix a typographical error.

Change-Id: Ie02f840baaa7c5056dfb281b2611c9d48525c51f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/61010
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/re2/re2.h b/re2/re2.h
index 1d82518..449162a 100644
--- a/re2/re2.h
+++ b/re2/re2.h
@@ -591,7 +591,7 @@
   // Replace(). E.g. if rewrite == "foo \\2,\\1", returns 2.
   static int MaxSubmatch(const StringPiece& rewrite);
 
-  // Append the "rewrite" string, with backslash subsitutions from "vec",
+  // Append the "rewrite" string, with backslash substitutions from "vec",
   // to string "out".
   // Returns true on success.  This method can fail because of a malformed
   // rewrite string.  CheckRewriteString guarantees that the rewrite will