Add grammatical article to “an ASCII letter”

Doc/library/re.rst

@@ -321,7 +321,7 @@ The special characters are:
 
 
 The special sequences consist of ``'\'`` and a character from the list below.
-If the ordinary character is not ASCII digit or ASCII letter, then the
+If the ordinary character is not an ASCII digit or an ASCII letter, then the
 resulting RE will match the second character.  For example, ``\$`` matches the
 character ``'$'``.
 
@@ -444,7 +444,7 @@ three digits in length.
    The ``'\u'`` and ``'\U'`` escape sequences have been added.
 
 .. versionchanged:: 3.6
-   Unknown escapes consisting of ``'\'`` and ASCII letter now are errors.
+   Unknown escapes consisting of ``'\'`` and an ASCII letter now are errors.
 
 
 .. seealso::
@@ -743,7 +743,7 @@ form.
       Unmatched groups are replaced with an empty string.
 
    .. versionchanged:: 3.6
-      Unknown escapes consisting of ``'\'`` and ASCII letter now are errors.
+      Unknown escapes consisting of ``'\'`` and an ASCII letter now are errors.
 
 
 .. function:: subn(pattern, repl, string, count=0, flags=0)

Misc/NEWS

@@ -56,7 +56,7 @@ Library
 - Issue #27029: Removed deprecated support of universal newlines mode from
   ZipFile.open().
 
-- Issue #27030: Unknown escapes consisting of ``'\'`` and ASCII letter in
+- Issue #27030: Unknown escapes consisting of ``'\'`` and an ASCII letter in
   regular expressions now are errors.  The re.LOCALE flag now can be used
   only with bytes patterns.