[fetch] Fix docstring typos/inaccuracy.
[fetch] Fix docstring typos/inaccuracy.

TBR=gbeaty@chromium.org, tandrii@chromium.org

Change-Id: I0d4cae099fc792df0b45d170523e95aa64579fd4
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1695808
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py