Fix LLVM build steps (#775)

Author: p-ouellette

README.md

@@ -55,6 +55,7 @@ LLVM can be built in the following way:
 ```
 cd llvm
 mkdir build
+cd build
 cmake -GNinja -DLLVM_ENABLE_RTTI=ON -DLLVM_ENABLE_EH=ON -DBUILD_SHARED_LIBS=ON -DCMAKE_BUILD_TYPE=Release -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_ENABLE_ASSERTIONS=ON -DLLVM_ENABLE_PROJECTS="llvm;clang" ../llvm
 ```