Style link in wrong password error

It was missing the class which gives it the correct colour and underline styles.

Also adds a little bit of spacing to make it look better.

app/main/views/sign_in.py

@@ -63,7 +63,7 @@ def sign_in():
         flash(Markup(
             (
                 f"The email address or password you entered is incorrect."
-                f" <a href={password_reset_url}>Forgotten your password?</a>"
+                f"&ensp;<a href={password_reset_url} class='govuk-link'>Forgotten your password?</a>"
             )
         ))