PARQUET-1111: Fix help for verify-release-candidate

The tool takes 2 parameters, version and rc-num. The help only printed a
single parameter.

Author: Lars Volker <lv@cloudera.com>

Closes #402 from lekv/p1111 and squashes the following commits:

7536709 [Lars Volker] PARQUET-1111: Fix help for verify-release-candidate
diff --git a/dev/release/verify-release-candidate b/dev/release/verify-release-candidate
index 9bdf2b0..d9c3690 100755
--- a/dev/release/verify-release-candidate
+++ b/dev/release/verify-release-candidate
@@ -71,7 +71,8 @@
      verify_rc_num="$2"
      ;;
 
-  *) echo "Usage: $0 RC_VERSION"
+  *) echo "Usage: $0 VERSION RC_NUM"
+     echo "Example to test 1.3.0-rc2: $0 1.3.0 2"
      exit 1
      ;;
 esac