Minor - add missing instruction (#11131)

The "list" instruction was missing, so the command didn't work properly

Author: Mir3605

examples/example-reflection/README.md

@@ -45,7 +45,7 @@ Output
 
 ### List all the methods of a service
   ```
-  $ grpcurl -plaintext localhost:50051 helloworld.Greeter
+  $ grpcurl -plaintext localhost:50051 list helloworld.Greeter
   ```
 Output
   ```