Update benchmark example output

This has been running hundreds of times faster since Rust 1.40.

Author: dtolnay

benches/bench-libproc-macro/README.md

@@ -4,7 +4,7 @@ Example output:
 $ cargo check --release
 
    Compiling bench-libproc-macro v0.0.0
-STRING: 8 millis
-TOKENSTREAM: 25721 millis
-    Finished release [optimized] target(s) in 26.15s
+STRING: 15 millis
+TOKENSTREAM: 97 millis
+    Finished release [optimized] target(s) in 1.06s
 ```