Update README.md (#232)

Fix error in command
diff --git a/README.md b/README.md
index 32b227f..6e3ed2f 100644
--- a/README.md
+++ b/README.md
@@ -102,7 +102,7 @@
 
 2. Publish the firebase hosted web release.
     * Log in to the account that has write access to `gallery-flutter-dev` with `firebase login`
-    * `flutter web build`
+    * `flutter build web`
     * `firebase deploy -P prod` to deploy to production (equivalent to `firebase deploy`).
     * `firebase deploy -P staging` to deploy to staging. Check with the team to see if the staging
        instance is currently used for a special purpose.