Fix an ambiguous commit hash.

Author: ericsnowcurrently

Doc/reference/executionmodel.rst

@@ -529,7 +529,7 @@ into Python in the context of a specific interpreter.
 
    In an ideal world, "Python runtime" would refer to what we currently
    call "interpreter".  However, it's been called "interpreter" at least
-   since introduced in 1997 (a027efa5b).
+   since introduced in 1997 (CPython:a027efa5b).
 
 Each interpreter completely encapsulates all of the non-process-global,
 non-thread-specific state needed for the Python runtime to work.