remove buck2 from mps readme

Summary:
for alpha+, we need to remove all buck2 commands and buck2 dependencies from static doc and github readmes.
This diff gets rid of the buck2 from  mps backend readme

Differential Revision: D59659285

Author: Gasoonjia

examples/apple/mps/README.md

@@ -28,8 +28,7 @@ Once we have the model binary file, then let's run it with the ExecuTorch runtim
 
 ```bash
 # Build and install executorch
-cmake -DBUCK2="$BUCK" \
-          -DCMAKE_INSTALL_PREFIX=cmake-out \
+cmake -DCMAKE_INSTALL_PREFIX=cmake-out \
           -DCMAKE_BUILD_TYPE=Release \
           -DEXECUTORCH_BUILD_SDK=ON \
           -DEXECUTORCH_ENABLE_EVENT_TRACER=ON \