Update Clang doc section for using a custom clang binary.

Give better instructions on how to check the toolchain.ninja file.

Change-Id: I259d29bf22b4f2858716f2a7e1c16dced2864e66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2889482
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#881979}

docs/clang.md

@@ -80,8 +80,8 @@ On Windows, for `clang_base_path` use something like this instead:
 clang_base_path = "c:/src/llvm-build"
 ```
 
-You can then run `head out/gn/toolchain.ninja` and check that the first to
-lines set `cc` and `cxx` to your clang binary. If things look good, run `ninja
+You can then look in `out/gn/toolchain.ninja` and check that the `rule cc` and
+`rule cxx` commands run your clang binary.  If things look good, run `ninja
 -C out/gn` to build.
 
 Chromium tries to be buildable with its currently pinned clang, and with clang