Fix android-cpu flag reference (#3943)
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 4a14cc0..5a0b756 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -86,7 +86,7 @@
* `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
