Update index.md (MystenLabs#1618)

Add dollar signs to command in Contribute index

doc/src/contribute/index.md

@@ -38,7 +38,7 @@ at [Install Sui](../build/install.md#source-code).
 
 Start by creating your own fork of the repo:
 ```bash
-gh repo fork https://github.com/mystenlabs/sui.git # or alternatively, clone your fork
+$ gh repo fork https://github.com/mystenlabs/sui.git # or alternatively, clone your fork
 cargo install --path sui/sui # put Sui CLI's in your PATH
 cd sui
 cargo build --all --all-targets # check that build works