Change instances of iTutorial to tutorial.

While the new ChromeVox tutorial was being developed, we had to keep
two tutorials around. The new tutorial was named iTutorial as a result.

Now that the old tutorial has been removed, replace all instances of
'iTutorial' with 'tutorial'.

Bug: N/A
Change-Id: I13324e722ef38d975249ce244adb87e37721d3da
AX-Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2673959
Commit-Queue: Akihiro Ota <akihiroota@chromium.org>
Reviewed-by: Josiah Krutz <josiahk@google.com>
Cr-Commit-Position: refs/heads/master@{#850405}