[typo] Fix minor typo in raise TypeError Closes #151

Fix #151

netjsonconfig/backends/base/backend.py

@@ -62,7 +62,7 @@ def _load(self, config):
                 pass
         if not isinstance(config, dict):
             raise TypeError(
-                'config block must be an istance ' 'of dict or a valid NetJSON string'
+                'config block must be an instance of dict or a valid NetJSON string'
             )
         return config