Survey: Fix file extension

Author: fhemberger

external/survey-2018/2018-nodejs-user-survey-raw-data.xslx renamed to external/survey-2018/2018-nodejs-user-survey-raw-data.xlsx

@@ -154,7 +154,7 @@ <h1>2018 Node.js User Survey Report</h1>
           </ul>
           <ul class="nav main-nav">
               <li><a href="2018-nodejs-user-survey-report.pdf">Download as PDF</a></li>
-              <li><a href="2018-nodejs-user-survey-raw-data.xslx">Get the raw data (Excel)</a></li>
+              <li><a href="2018-nodejs-user-survey-raw-data.xlsx">Get the raw data (Excel)</a></li>
           </ul>
           <ul class="back-to-top _muted">
               <li>