tutorial: Fix types in gettimeofday example. Closes #1657

Author: brson

doc/tutorial.md

@@ -2334,17 +2334,19 @@ microsecond-resolution timer.
 
 ~~~~
 use std;
-type timeval = {mutable tv_sec: u32,
-                mutable tv_usec: u32};
+type timeval = {mutable tv_sec: uint,
+                mutable tv_usec: uint};
 #[nolink]
 native mod libc {
     fn gettimeofday(tv: *timeval, tz: *()) -> i32;
 }
 fn unix_time_in_microseconds() -> u64 unsafe {
-    let x = {mutable tv_sec: 0u32, mutable tv_usec: 0u32};
+    let x = {mutable tv_sec: 0u, mutable tv_usec: 0u};
     libc::gettimeofday(ptr::addr_of(x), ptr::null());
     ret (x.tv_sec as u64) * 1000_000_u64 + (x.tv_usec as u64);
 }
+
+# fn main() { assert #fmt("%?", unix_time_in_microseconds()) != ""; }
 ~~~~
 
 The `#[nolink]` attribute indicates that there's no native library to link