Make the step of set extra environment variables work for both zsh and bash (#112)

Update contributing.md

Author: wykrrr

docs/docs/about/contributing.md

@@ -29,7 +29,7 @@ We love contributions from our community! This guide explains how to get involve
 
 -   (Optional) Before running a specific example, set extra environment variables, for exposing extra traces, allowing dev UI, etc.
     ```bash
-    . .env.lib_debug
+    . ./.env.lib_debug
     ```
 
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