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

diff --git a/Parser/string_parser.c b/Parser/string_parser.c
index 627879e..2c35da5 100644
--- a/Parser/string_parser.c
+++ b/Parser/string_parser.c
@@ -385,7 +385,7 @@
         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] = '(';