bpo-38619: Update the documentation for UUID.hex (GH-29830)

Explicitly state that it is lowercase.

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
(cherry picked from commit 5d53cf3)

Co-authored-by: 180909 <wjh180909@gmail.com>

Doc/library/uuid.rst

@@ -118,7 +118,7 @@ which relays any information about the UUID's safety, using this enumeration:
 
 .. attribute:: UUID.hex
 
-   The UUID as a 32-character hexadecimal string.
+   The UUID as a 32-character lowercase hexadecimal string.
 
 
 .. attribute:: UUID.int