Fix non-exiting link to TripleO-common bugs

Non-existing link: https://bugs.launchpad.net/tripleo-common
New corrected link: https://bugs.launchpad.net/tripleo

Change-Id: I9d9d4e3b67da02be60530493ce12b7e3878130a0
Closes-bug: 1805109

CONTRIBUTING.rst

@@ -13,4 +13,4 @@ Pull requests submitted through GitHub will be ignored.
 
 Bugs should be filed on Launchpad, not GitHub:
 
-   https://bugs.launchpad.net/tripleo-common
+   https://bugs.launchpad.net/tripleo