update benchmark readme (#72620)

Documents that version 2.0 of the async profiler doesn't seem to work
with jmh. Fixes some syntax in another profiling example.

benchmarks/README.md

@@ -102,12 +102,15 @@ If you want to disassemble a single method do something like this:
 gradlew -p benchmarks run --args ' MemoryStatsBenchmark -jvmArgs "-XX:+UnlockDiagnosticVMOptions -XX:CompileCommand=print,*.yourMethodName -XX:PrintAssemblyOptions=intel"
 ```
 
-If you want `perf` to find the hot methods for you then do add `-prof:perfasm`.
+If you want `perf` to find the hot methods for you then do add `-prof perfasm`.
 
 ## Async Profiler
 
 Note: Linux and Mac only. Sorry Windows.
 
+IMPORTANT: The 2.0 version of the profiler doesn't seem to be with compatible
+with JMH as of 2021-04-30.
+
 The async profiler is neat because it does not suffer from the safepoint
 bias problem. And because it makes pretty flame graphs!