Note that ninja = false goes under [llvm]

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
+You should install ninja, or set ninja=false in config.toml under the [llvm] section.
 "
                 );
                 std::process::exit(1);