Make example function in comment more idiomatic

src/libcore/result.rs

@@ -34,13 +34,11 @@
 //! enum Version { Version1, Version2 }
 //!
 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
-//!     if header.len() < 1 {
-//!         return Err("invalid header length");
-//!     }
-//!     match header[0] {
-//!         1 => Ok(Version::Version1),
-//!         2 => Ok(Version::Version2),
-//!         _ => Err("invalid version")
+//!     match header.get(0) {
+//!         None => Err("invalid header length"),
+//!         Some(&1) => Ok(Version::Version1),
+//!         Some(&2) => Ok(Version::Version2),
+//!         Some(_) => Err("invalid version")
 //!     }
 //! }
 //!