submodule documentation: fix ACL refspec

Change-Id: I53149b082a9a8b3eacaf950fa3004e4f124af042
Signed-off-by: Stefan Beller <sbeller@google.com>
diff --git a/Documentation/user-submodules.txt b/Documentation/user-submodules.txt
index 8f75bf1..977cc80 100644
--- a/Documentation/user-submodules.txt
+++ b/Documentation/user-submodules.txt
@@ -153,14 +153,14 @@
 'stable':
 ====
   [allowSuperproject "<superproject>"]
-    refs/heads/*:refs/heads/*
+    refs = refs/heads/*:refs/heads/*
 ====
 
 If you want to enable a branch to be subscribed to any other branch of
 the superproject, omit the second part of the RefSpec:
 ====
   [allowSuperproject "<superproject>"]
-    refs/heads/<submodule-branch>
+    refs = refs/heads/<submodule-branch>
 ====
 
 === Subscription Limitations