add image alt attribute (#305)

Author: paulodero

pages/getting-started/learning-odata-on-postman.html

@@ -22,7 +22,7 @@ <h2>How to Learn OData on Postman</h2>
 <div class="row">
     <div class="column">
         <img src="/assets/postmanCollection.jpg" width="800" height="490" onclick="openModal();currentSlide(1)"
-        onenter id="postmanCollection-small" class="hover-shadow cursor" tabindex="0">
+        onenter id="postmanCollection-small" class="hover-shadow cursor" tabindex="0" alt="Postman collection">
     </div>
 </div>