Remove markdown in codespace welcome.txt

I wrongly assumed markdown would work here.

.devcontainer/welcome.txt

@@ -1,6 +1,5 @@
 👋 Welcome to the Gemini API Cookbook! This repo is a collection of all the guides and examples for the Gemini API.
-    - To get started, make sure you have an API Key from [Google AI
-      studio](https://aistudio.google.com/app/apikey)
+    - To get started, make sure you have an API Key from Google AI studio: https://goo.gle/aistudio-key
     - Note that the notebooks were written to run in Google Colab, so they may
       not always work in Codespaces
     - Check out the 'README.md' file to know more