Correct wording about the content of OWNERS files in //third_party.

The wording in //docs/adding_to_third_party.md was confusing people.
It could be read as saying that an OWNERS file needed to have two
chromium.org email addresses in it, but it should have said just
two committers instead. This fixes that.

Change-Id: Ie03bc2bda626f3226a18a0d1112a04a8619b0dcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5528138
Reviewed-by: Joey Scarr <jsca@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/main@{#1298456}