_modules/prometheus: replace 'expr_form' with 'tgt_type'

_modules/prometheus.py

 def prometheus_targets_for_grain(key):
-    values = __salt__['mine.get']('*', 'grains.items', expr_form='glob').values()
+    values = __salt__['mine.get']('*', 'grains.items', tgt_type='glob').values()
     return [ remote_grains[key] for remote_grains in sorted(values) if key in remote_grains ]