uv: fix doc test

[Doc tests can't use crate internal APIs unfortunately.][internal-doc]
I really want them to be able to, but for now, mark such tests as
`ignore`.

I was motivated to do this because it otherwise breaks `cargo t --all`
for me.

[internal-doc]: rust-lang/rust#50784

Author: BurntSushi

crates/uv/src/shell.rs

@@ -60,7 +60,7 @@ impl Shell {
     ///
     /// # Examples
     ///
-    /// ```
+    /// ```ignore
     /// use crate::shells::Shell;
     ///
     /// assert_eq!(Shell::from_shell_path("/bin/bash"), Some(Shell::Bash));