refactor: remove yarn from uninstall help text

At the time of writing this, uninstalling this tool is not supported:

```
➜ volta uninstall yarn
error: Uninstalling yarn is not supported yet.
```

Author: MrChocolatine

src/command/uninstall.rs

@@ -9,7 +9,7 @@ use crate::command::Command;
 
 #[derive(StructOpt)]
 pub(crate) struct Uninstall {
-    /// The tool to uninstall, e.g. `yarn`, or <package>
+    /// The tool to uninstall, e.g. <package>
     tool: String,
 }