Fix four typos on devstack documentation

behaviour => behavior
mechansim => mechanism
glustfs => glusterfs
pluggin => plugin

Change-Id: I3de6740e5d4b2b55009614007767458124036b75
Closes-Bug: #1478538
diff --git a/HACKING.rst b/HACKING.rst
index a40af54..6bd24b0 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -355,7 +355,7 @@
 
 * **Should this be upstream** -- DevStack generally does not override
   default choices provided by projects and attempts to not
-  unexpectedly modify behaviour.
+  unexpectedly modify behavior.
 
 * **Context in commit messages** -- DevStack touches many different
   areas and reviewers need context around changes to make good