Fix trivial typo in the PEG string parser (GH-21508)

(cherry picked from commit 0275e0452a773976827c2b9bd1e598ee08e2d7f5)

Co-authored-by: Eric V. Smith <ericvsmith@users.noreply.github.com>
diff --git a/Parser/pegen/parse_string.c b/Parser/pegen/parse_string.c
index d762e65..7b02bdd 100644
--- a/Parser/pegen/parse_string.c
+++ b/Parser/pegen/parse_string.c
@@ -390,7 +390,7 @@
         return NULL;
     }
 
-    // The parentheses are needed in order to allow for leading whitespace withing
+    // The parentheses are needed in order to allow for leading whitespace within
     // the f-string expression. This consequently gets parsed as a group (see the
     // group rule in python.gram).
     str[0] = '(';