Fix name of commons-net dependency

The name was changed from commons_net to commons-net in gerrit
core change I1e75690fe.

Change-Id: I7688f0e09441c719e318ea51763435acf5f17a1a
diff --git a/BUILD b/BUILD
index 1c3873a..cc18db3 100644
--- a/BUILD
+++ b/BUILD
@@ -23,6 +23,6 @@
     tags = ["automerger"],
     deps = PLUGIN_DEPS + PLUGIN_TEST_DEPS + [
         ":automerger__plugin",
-        "@commons_net//jar",
+        "@commons-net//jar",
     ],
 )