PEP 426: fix typo

Author: MonoS

peps/pep-0426.rst

@@ -630,7 +630,7 @@ characters:
 
 Source labels MUST start and end with an ASCII letter or digit.
 
-A regular expression to rnforce these constraints (when run with
+A regular expression to enforce these constraints (when run with
 ``re.IGNORECASE``) is::
 
     ^([A-Z0-9]|[A-Z0-9][A-Z0-9._-+]*[A-Z0-9])$