Add doc alias for chdir to std::env::set_current_dir

Searching for `chdir` in the Rust documentation produces no useful
results.

library/std/src/env.rs

@@ -61,6 +61,7 @@ pub fn current_dir() -> io::Result<PathBuf> {
 /// assert!(env::set_current_dir(&root).is_ok());
 /// println!("Successfully changed working directory to {}!", root.display());
 /// ```
+#[doc(alias = "chdir")]
 #[stable(feature = "env", since = "1.0.0")]
 pub fn set_current_dir<P: AsRef<Path>>(path: P) -> io::Result<()> {
     os_imp::chdir(path.as_ref())