Keep snippet vars definitions in a single format across all snippets and documentation.

The format is:

# var-name: value

Previously snippets used a mixture of the following formatting:

#var-name: value
#var-name : value
# var-name: value
# var-name : value

snippets/nxml-mode/header/h2

@@ -1,5 +1,5 @@
-#contributor : Anders Bach Nielsen <abachn@abachn.net>
-#name : <h2>...</h2>
+# contributor: Anders Bach Nielsen <abachn@abachn.net>
+# name: <h2>...</h2>
 # key: h2
 # --
 <h2>$1</h2>