Fix android-cpu flag reference (flutter#3943)

CONTRIBUTING.md

@@ -86,7 +86,7 @@ Run the following steps, from the `src` directory created in the steps above:
 
 * `git pull upstream master` in `src/flutter` to update the Flutter Engine repo.
 * `gclient sync` to update your dependencies.
-* `./flutter/tools/gn --android --unoptimized` to prepare your build files (or `--android --android_cpu [x86|x64] --unoptimized` for x86/x64 emulators) .
+* `./flutter/tools/gn --android --unoptimized` to prepare your build files (or `--android --android-cpu [x86|x64] --unoptimized` for x86/x64 emulators) .
 * `ninja -C out/android_debug_unopt` to actually build the Android binary (or `out/android_debug_unopt_x64 for x86/x64 emulators).
 
 This builds a debug-enabled ("unoptimized") binary configured to run Dart in