Fix typo

README used EXPECT_CALL(...).WillByDefault()
when it should have been .WillRepeatedly()

WillByDefault() will still print warnings.
(Also it requires ON_CALL, not EXPECT_CALL)

Change-Id: I91c1808a993678ee3842c5feb41d7c4b020e16cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6770640
Commit-Queue: Alison Gale <agale@chromium.org>
Commit-Queue: Dana Fried <dfried@chromium.org>
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Alison Gale <agale@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1488876}