Fix regression of #821 in puppet of show_diff

Issue #821 fix: 1382576100
Regressed since: 6912ca0aca

Issue: #821

system/puppet.py

@@ -175,7 +175,7 @@ def main():
         if p['puppetmaster']:
             cmd += " --server %s" % pipes.quote(p['puppetmaster'])
         if p['show_diff']:
-            cmd += " --show-diff"
+            cmd += " --show_diff"
         if p['environment']:
             cmd += " --environment '%s'" % p['environment']
         if module.check_mode: