gh-141007: update string module source code link (GH-141008)

In 3.14, the former string.py became `__init__.py` within a new `string` directory that also contains a new submodule file, `templatelib.py`.
(cherry picked from commit fa02422)

Co-authored-by: alex <30386655+alexomics@users.noreply.github.com>

Author: alexomics

Doc/library/string.rst

@@ -4,7 +4,7 @@
 .. module:: string
    :synopsis: Common string operations.
 
-**Source code:** :source:`Lib/string.py`
+**Source code:** :source:`Lib/string/__init__.py`
 
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