Fix identifiers in broadcast protocol for correctness verification

examples/broadcast.pp

 { "title": "Broadcast Protocol",
-  "states": ["true", "false"],
+  "states": ["_true", "_false"],
   "transitions": [{ "name": "true,false->true,true",
-                    "pre": ["true", "false"],
-                    "post": ["true", "true"]
+                    "pre": ["_true", "_false"],
+                    "post": ["_true", "_true"]
                 }
                ],
-  "initialStates": ["true", "false"],
-  "trueStates": ["true"],
-  "predicate": "true >= 1"
+  "initialStates": ["_true", "_false"],
+  "trueStates": ["_true"],
+  "predicate": "_true >= 1"
 }