Rename Error::IO to Error::Io (#5174)

## Summary

I believe this is by convention (see, e.g., in Rust itself:
https://github.com/search?q=repo%3Arust-lang%2Frust+%2F%28%3F-i%29Io%2F&type=code).

crates/uv-python/src/discovery.rs

@@ -149,7 +149,7 @@ pub enum PythonSource {
 #[derive(Error, Debug)]
 pub enum Error {
     #[error(transparent)]
-    IO(#[from] io::Error),
+    Io(#[from] io::Error),
 
     /// An error was encountering when retrieving interpreter information.
     #[error(transparent)]