Update unclear instructions on how to checkout code with no history

`fetch chromium --no-history` was the intuitive thing to try, which errors.

Change-Id: Ie0d3befe87730bffded40c20611f77ed30ed5006
Reviewed-on: https://chromium-review.googlesource.com/c/1299930Reviewed-by: Paul Irish <paulirish@chromium.org>
Commit-Queue: Paul Irish <paulirish@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603114}

docs/mac_build_instructions.md

@@ -63,8 +63,8 @@ dependencies.
 $ fetch chromium
 ```
 
-If you don't want the full repo history, you can save a lot of time by
-adding the `--no-history` flag to `fetch`.
+If you don't need the full repo history, you can save time by using
+`fetch --no-history chromium`.
 
 Expect the command to take 30 minutes on even a fast connection, and many
 hours on slower ones.