Fix typo in devstack/stack.sh

Change-Id: Ie13f1ae7fb5a46bb654aa3ab526933107c485b0b
diff --git a/stack.sh b/stack.sh
index d83952a..0e22804 100755
--- a/stack.sh
+++ b/stack.sh
@@ -23,7 +23,7 @@
 
 # check if someone has invoked with "sh"
 if [[ "${POSIXLY_CORRECT}" == "y" ]]; then
-    echo "You appear to be running bash in POSIX compatability mode."
+    echo "You appear to be running bash in POSIX compatibility mode."
     echo "devstack uses bash features. \"./stack.sh\" should do the right thing"
     exit 1
 fi