[bugfix] Fix a11y property warning from authorization page (#4166)

Use correct input-field identifier for authorization page email label. This seemed to be only instance where label was broken, other labels either had correct for-identifier or enclosed the input which they are labeling.

closes #4165

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4166
Co-authored-by: Iivari Äikäs <iivari.aikas@gmail.com>
Co-committed-by: Iivari Äikäs <iivari.aikas@gmail.com>

web/template/sign-in.tmpl

@@ -23,7 +23,7 @@
         <h2 id="sign-in">Sign in</h2>
         <form action="/auth/sign_in" method="POST">
             <div class="labelinput">
-                <label for="email">Email</label>
+                <label for="username">Email</label>
                 <input
                     type="email"
                     name="username"