Update externals_login_form.html

Added Alt Tag information to resolve the Image missing a text alternative - Siteimprove

Author: Johnsonjesuraj

django_airavata/apps/auth/templates/django_airavata_auth/partials/externals_login_form.html

@@ -16,7 +16,7 @@ <h5 class="card-title">{{ title|default:"Log in with your existing organizationa
                 {% endif %}
                     class="btn btn-primary btn-block mt-3">
                   {% if external.logo %}
-                  <img src="{% static external.logo %}"/>
+                  <img src="{% static external.logo %}" alt="Sign in with {{ external.name }}"/>
                   {% endif %}
                         Sign in with {{ external.name }}
                 </a>