docs: remove Ruby gem from root install list

Remove the RubyGems installation command from the root README SDK overview while leaving the Ruby SDK's dedicated documentation unchanged.

Author: appcypher

README.md

@@ -59,10 +59,6 @@
 > ```sh
 > go get github.com/superradcompany/microsandbox/sdk/go    # 🐹 Go
 > ```
->
-> ```sh
-> gem install microsandbox                                 # 💎 Ruby
-> ```
 #### <img height="14" src="https://octicons-col.vercel.app/download/A770EF">&nbsp;&nbsp;Install the CLI
 
 > Boot a microVM in a single command: