Register Button name (withastro#8129)

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

src/content/docs/en/guides/backend/supabase.mdx

@@ -259,7 +259,7 @@ import Layout from "../layouts/Layout.astro";
     <input type="email" name="email" id="email" />
     <label for="password">Password</label>
     <input type="password" name="password" id="password" />
-    <button type="submit">Login</button>
+    <button type="submit">Register</button>
   </form>
 </Layout>
 ```