i18n input field on login page (LMS-2159)

lms/templates/login.html

@@ -130,7 +130,7 @@
         <ol class="list-input">
           <li class="field required text" id="field-email">
             <label for="email">${_('E-mail')}</label>
-            <input class="" id="email" type="email" name="email" value="" placeholder="example: username@domain.com" required aria-required="true" aria-described-by="email-tip" />
+            <input class="" id="email" type="email" name="email" value="" placeholder="${_('example: username@domain.com')}" required aria-required="true" aria-described-by="email-tip" />
             <span class="tip tip-input" id="email-tip">${_("This is the e-mail address you used to register with {platform}").format(platform=platform_name)}</span>
           </li>
           <li class="field required password" id="field-password">