docs: apache2 htaccess, avoid scriptname in URLs

adjust .htaccess example to show 404 on pretty URLs, when
$showScriptName is false.

fixes https://github.com/yiisoft/yii2-app-basic/issues/109

docs/guide/start-installation.md

@@ -200,6 +200,9 @@ DocumentRoot "path/to/basic/web"
     # Otherwise forward the request to index.php
     RewriteRule . index.php
 
+    # if $showScriptName is false in UrlManager, do not allow accessing URLs with script name
+    RewriteRule ^index.php/ - [L,R=404]
+
     # ...other settings...
 </Directory>
 ```