Update llvm install command [skip ci]

cpp/README.md

@@ -323,7 +323,7 @@ You can also fix any formatting errors automatically:
 These commands require `clang-format-6.0` (and not any other version).
 You may find the required packages at http://releases.llvm.org/download.html
 or use the Debian/Ubuntu APT repositories on https://apt.llvm.org/. On macOS
-with [Homebrew][1] you can get it via `brew install llvm@5`.
+with [Homebrew][1] you can get it via `brew install llvm@6`.
 
 ## Checking for ABI and API stability