[clang] Add build type to LibASTMatchersTutorial.rst cmake (#76301)

Add the required CMAKE_BUILD_TYPE to the cmake configuration line.

We drop the comment about enabling tests, since it is already implied
and doesn't add any additional context.

clang/docs/LibASTMatchersTutorial.rst

@@ -50,7 +50,7 @@ Okay. Now we'll build Clang!
 
       cd ~/clang-llvm
       mkdir build && cd build
-      cmake -G Ninja ../llvm -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DLLVM_BUILD_TESTS=ON  # Enable tests; default is off.
+      cmake -G Ninja ../llvm -DLLVM_ENABLE_PROJECTS="clang;clang-tools-extra" -DCMAKE_BUILD_TYPE=Release -DLLVM_BUILD_TESTS=ON
       ninja
       ninja check       # Test LLVM only.
       ninja clang-test  # Test Clang only.