Fix android runner command in llama2 readme (#2891)

Summary:
Pull Request resolved: #2891

att

Created from CodeHub with https://fburl.com/edit-in-codehub

Reviewed By: mcr229

Differential Revision: D55825424

fbshipit-source-id: d98708c7ebc7138ce78854d5e03339150dc63406

Author: kimishpatel

examples/models/llama2/README.md

@@ -196,6 +196,8 @@ cmake  -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
     -DEXECUTORCH_BUILD_OPTIMIZED=ON \
     -Bcmake-out-android/examples/models/llama2 \
     examples/models/llama2
+
+cmake --build cmake-out-android/examples/models/llama2 -j16 --config Release
 ```
 
 **2. Run on Android via adb shell**