Correct a typo in the document

This patch changes 'Note that is is'
to 'Note that it is' in doc/source/
configuration/kolla.rst

Change-Id: Ide2e9a7582b78b25eac3462dc3477faf0ee66d38

doc/source/configuration/kolla.rst

@@ -274,7 +274,7 @@ This will result in Kayobe adding the following configuration to
    location = https://git.example.com/ironic
    reference = downstream
 
-Note that is is currently necessary to include the Bifrost source location if
+Note that it is currently necessary to include the Bifrost source location if
 using a seed.
 
 Plugins & additions