Remove directive formatting from NOTE marker (#499)

Before this commit, this snippet formatted a NOTE marker as a snippet directive,
as `NOTE:`. As `NOTE` isn't a directive, this triggered a directive warning.

snippets/python-mode/function_docstring

@@ -2,7 +2,7 @@
 # name: function_docstring
 # key: fd
 # group: definitions
-# NOTE: Use minimum indentation, because Emacs 25+ doesn't dedent docstrings.
+# NOTE Use minimum indentation, because Emacs 25+ doesn't dedent docstrings.
 # --
 def ${1:name}($2):
  \"\"\"$3