Updates fixed-size suffix

Compiler gives the following warning:
`warning: the `u` suffix on integers is deprecated; use `us` or one of the fixed-sized suffixes`
And also the errror:
`error: mismatched types: expected `u64`, found `usize` (expected u64, found usize)`

Changing the suffix to `u64` results in a successful `cargo run` outputting the desired `Versions compared successfully!`

src/doc/intro.md

@@ -106,9 +106,9 @@ use semver::Version;
 
 fn main() {
     assert!(Version::parse("1.2.3") == Ok(Version {
-        major: 1u,
-        minor: 2u,
-        patch: 3u,
+        major: 1u64,
+        minor: 2u64,
+        patch: 3u64,
         pre: vec!(),
         build: vec!(),
     }));