title: Remove the last comma in a string
body: <p>I have a data.frame. It looks like this:</p>
  
  <pre><code>name    state
  Lily    NY
  Tom     NY,NJ,
  John    PA,NJ
  David   SC,PA,NY,
  Jim     FL,PA
  ......
  </code></pre>
  
  <p>There are more than 100 rows. I just want to remove the last comma in each string if there is. My goal is not to remove all the last character. </p>