[doc] Correct mapping JSON example

A sample mapping does not contain valid JSON and will fail to validate.
Replace the "=" with ":" to correct the example.

Change-Id: I0c6c64ad131da1120d6acad903d5388e30eb7604

doc/source/federation/mapping_combinations.rst

@@ -618,7 +618,7 @@ names we have in the Identity Provider. It will map any user with the name
 .. code-block:: javascript
 
     {
-        rules = [
+        "rules": [
             {
                 "local": [
                     {