Clarify the root directory comment

A contributor had an issue when ran script changing the directory
name after clone. This was discussed on the etcd slack channel.

scripts/genproto.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 #
 # Generate all etcd protobuf bindings.
-# Run from repository root.
+# Run from repository root directory named etcd.
 #
 set -e