improved explanation of trialing with composer (#99)

bibtexbrowser-documentation.wiki

@@ -545,7 +545,8 @@ In your project's `composer.json`, just add this <pre>"monperrus/bibtexbrowser":
 }
 </pre>
 
-With composer, a test server can be started with this command:
+With composer, a test server can be started with the following commands:
+<pre>cd vendor/monperrus/bibtexbrowser/</pre>
 <pre>composer serve</pre>
 Which runs this script:
 <pre>php -S localhost:29896 bibtexbrowser.php</pre>