Upgrade/Install: Add missing `</form>` tag in auto-updates settings form.

Props imath, ahortin, dd32, afragen.
Fixes #51598. See #50907.

git-svn-id: https://develop.svn.wordpress.org/trunk@49274 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/update-core.php

@@ -382,6 +382,7 @@ function core_auto_updates_settings() {
 		<p>
 			<input id="core-auto-updates-settings" class="button" type="submit" value="<?php esc_attr_e( 'Save' ); ?>" name="core-auto-updates-settings" />
 		</p>
+	</form>
 	<?php
 }