Fix mistyped variable name obj_type in WoldObj.decode method (#89)

* add .idea to .gitignore

* fix typo in obj_type

.gitignore

@@ -6,3 +6,4 @@ trained_models
 # PyPI
 build/*
 dist/*
+.idea/

gym_minigrid/minigrid.py

@@ -142,7 +142,7 @@ class WorldObj:
         elif obj_type == 'lava':
             v = Lava()
         else:
-            assert False, "unknown object type in decode '%s'" % objType
+            assert False, "unknown object type in decode '%s'" % obj_type
 
         return v