Correct the generate_source.py file name

It is generate_source.py, not generate_sources.py.

Change-Id: I2b44dd0ac343d1eaa91b48a4402406009c82b052
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419588Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808927}

third_party/dav1d/README.chromium

@@ -22,8 +22,8 @@ First have clang (//third_party/llvm-build/Release+Asserts/bin), clang-cl
 and meson (https://github.com/mesonbuild/meson/releases) in your path. Then you
 can run ./generate_configs.py
 
-Next you should update the dav1d_generated.gni file, to that run the
-./generate_sources.py command.
+Next you should update the dav1d_generated.gni file, to do that run the
+./generate_source.py command.
 
 Hopefully all that works and then you can roll DEPS to the new revision with
 all the updated configuration files. If not, you'll need to debug the bots :(