make (not yet actually used) doctest actually use the crate, and fix a comment

cargo-miri/bin.rs

@@ -140,7 +140,7 @@ fn xargo_check() -> Command {
     Command::new(env::var_os("XARGO_CHECK").unwrap_or_else(|| OsString::from("xargo-check")))
 }
 
-/// Execute the command if it fails, fail this process with the same exit code.
+/// Execute the command. If it fails, fail this process with the same exit code.
 /// Otherwise, continue.
 fn exec(mut cmd: Command) {
     let exit_status = cmd.status().expect("failed to run command");

test-cargo-miri/src/lib.rs

@@ -1,6 +1,6 @@
 /// Doc-test test
 /// ```rust
-/// assert!(true);
+/// assert!(cargo_miri_test::make_true());
 /// ```
 pub fn make_true() -> bool {
     true