Update installation.md to use branch main (#243)

With the switch from master to main, the default cargo install command no longer works. It requires the branch to be manually specified.

docs/src/cli/installation.md

@@ -14,7 +14,7 @@ cargo install selene
 
 **If you want the most up to date version of selene**
 ```
-cargo install --git https://github.com/Kampfkarren/selene selene
+cargo install --branch main --git https://github.com/Kampfkarren/selene selene
 ```
 
 ### Roblox developers