docs: fix tar command in installation instructions

The installation instructions had 'tar cvfz' which creates an archive,
but it should be 'tar xvfz' to extract the downloaded archive.

- Changed 'cvfz' (create) to 'xvfz' (extract)

Author: RamiR

docs/source/intro.rst

@@ -11,7 +11,7 @@ install the module system-wide run
 
 .. code-block:: bash
 
-   $ tar cvfz python-sqlparse-VERSION.tar.gz
+   $ tar xvfz python-sqlparse-VERSION.tar.gz
    $ cd python-sqlparse/
    $ sudo python setup.py install