Use singular they. Thanks to Mark Summerfield.

birkenfeld · birkenfeld · commit 516637504502 · 2011-05-13T06:55:28.000+02:00
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
@@ -814,7 +814,7 @@ always available.
    Python.
 
    The highest possible limit is platform-dependent.  A user may need to set the
-   limit higher when she has a program that requires deep recursion and a platform
+   limit higher when they have a program that requires deep recursion and a platform
    that supports a higher limit.  This should be done with care, because a too-high
    limit can lead to a crash.
 
