Update headings-and-labels.html

magnify glass -> magnifying glass
(assuming this is the more common term?)

understanding/20/headings-and-labels.html

@@ -32,7 +32,7 @@ <h2>Intent of Headings and Labels</h2>
       </p>
        <p>Labels of form controls are usually text-based. In some cases, images can serve as descriptive labels without additional text. In these cases, authors would need to ensure that the image and its use as label in a particular context can be considered conventional and widely understood. 
          Note that the same image can be interpreted differently in different contexts. However, it can still be considered descriptive if its use is commonly understood in each context. For example, when accompanying a text field, a loupe or magnifying glass icon is commonly interpreted as indicating the field is for entering and submitting a search query. 
-         Placed on or near another image, a loupe or magnify glass icon is commonly interpreted as a means to view a magnified version of the image (for instance, acting as a mechanism to zoom into the image, or opening a full-sized image in a new window).</p>
+         Placed on or near another image, a loupe or magnifying glass icon is commonly interpreted as a means to view a magnified version of the image (for instance, acting as a mechanism to zoom into the image, or opening a full-sized image in a new window).</p>
 
       <p>This Success Criterion does not require headings or labels. This Success Criterion
          requires that if headings or labels are provided, they be descriptive. This Success Criterion also