display help for commands that have Summary but not Usage

A command doesn't have to specify Usage docs if it doesn't accept any
arguments. The default usage for a command will be printed as:

    Usage: rbenv ${command}

libexec/rbenv-rehash

@@ -1,6 +1,5 @@
 #!/usr/bin/env bash
 # Summary: Rehash rbenv shims (run this after installing binaries)
-# Usage: rbenv rehash
 
 set -e
 [ -n "$RBENV_DEBUG" ] && set -x