Fix forgot password modal for theming

Use the `PLATFORM_NAME` setting instead of "edX"

lms/templates/forgot_password_modal.html

@@ -18,7 +18,7 @@
             <li class="field required text" id="field-email">
               <label for="pwd_reset_email">Your E-mail Address</label>
               <input class="" id="pwd_reset_email" type="email" name="email" value="" placeholder="example: username@domain.com" />
-              <span class="tip tip-input">This is the email address you used to register with edX</span>
+              <span class="tip tip-input">This is the email address you used to register with ${settings.PLATFORM_NAME}</span>
             </li>
           </ol>
         </fieldset>