DOC: fix details the value of include-system-site-packages

Now matches both site.rst and the actual implementation.

Author: tacaswell

Lib/site.py

@@ -18,7 +18,7 @@
 it is also checked for site-packages (sys.base_prefix and
 sys.base_exec_prefix will always be the "real" prefixes of the Python
 installation). If "pyvenv.cfg" (a bootstrap configuration file) contains
-the key "include-system-site-packages" set to anything other than "false"
+the key "include-system-site-packages" is set to  "true"
 (case-insensitive), the system-level prefixes will still also be
 searched for site-packages; otherwise they won't.