Fix that the path of functional test

before change:
$ tox -e functional -- --regex functional.tests.compute.v2.test_server
after change:
$ tox -e functional -- --regex tests.functional.compute.v2.test_server

the test unit path document should be change the above line.

(fixed wrong letter)

Change-Id: I49674fb0d56ee65c1f6328b9d960b16876173e2d

Author: RyanKor

doc/source/contributor/developing.rst

@@ -88,7 +88,7 @@ To run a specific functional test:
 
 .. code-block:: bash
 
-    $ tox -e functional -- --regex functional.tests.compute.v2.test_server
+    $ tox -e functional -- --regex tests.functional.compute.v2.test_server
 
 Running with PDB
 ~~~~~~~~~~~~~~~~