add missing period; thanks to Gaston Fiore from docs@

Doc/tutorial/modules.rst

@@ -248,7 +248,7 @@ modules are built into the interpreter; these provide access to operations that
 are not part of the core of the language but are nevertheless built in, either
 for efficiency or to provide access to operating system primitives such as
 system calls.  The set of such modules is a configuration option which also
-depends on the underlying platform For example, the :mod:`winreg` module is only
+depends on the underlying platform.  For example, the :mod:`winreg` module is only
 provided on Windows systems. One particular module deserves some attention:
 :mod:`sys`, which is built into every Python interpreter.  The variables
 ``sys.ps1`` and ``sys.ps2`` define the strings used as primary and secondary