fix typos in "About State Descriptions" details

templates/sim-a11y-view.html

@@ -215,7 +215,7 @@ <h3>State Descriptions for {{PHET_SIM_TITLE}}</h3>
           State Descriptions are a combination of static state descriptions (descriptions that never change) and dynamic
           state descriptions (descriptions that update silently as the simulation changes). Together, they form a
           complete description of the current state at any given moment. These are the descriptions that a screen reader
-          users can read through or review as they feel the need when they accessing a simulation (i.e., not the A11y
+          user can read through or review as they feel the need when they are accessing a simulation (i.e., not the A11y
           View tool).
         </details>
         <div id="dom-copy-container" class="dom_copy_container" role="presentation"></div>