Update URL that points at a blank page.

Looks like the URL changed where #workflow became workflow.html

Author: sg-

.github/pull_request_template.md

@@ -3,7 +3,7 @@
 <!-- 
     Required
     Add here detailed changes summary, testing results, dependencies 
-    Good example: https://os.mbed.com/docs/latest/reference/guidelines.html#workflow (Pull request template)
+    Good example: https://os.mbed.com/docs/latest/reference/workflow.html (Pull request template)
 -->