#12389: fix missing space at sentence end.

Doc/library/urllib.rst

@@ -210,7 +210,7 @@ Utility functions
 
    Replace special characters in *string* using the ``%xx`` escape. Letters,
    digits, and the characters ``'_.-'`` are never quoted. By default, this
-   function is intended for quoting the path section of the URL.The optional
+   function is intended for quoting the path section of the URL. The optional
    *safe* parameter specifies additional characters that should not be quoted
    --- its default value is ``'/'``.