Improve error message

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

Author: jyn514

src/bootstrap/lib.rs

@@ -1366,7 +1366,7 @@ impl Build {
                 eprintln!(
                     "
 Couldn't find required command: ninja
-You should install ninja, or set ninja=false in config.toml under the [llvm] section.
+You should install ninja, or set `ninja=false` in config.toml in the `[llvm]` section.
 "
                 );
                 std::process::exit(1);