Fix wrong docs about default image upload location
Fix wrong docs about default image upload location

We wrongly state that the default image upload location is imgur. This
is no longer true, but got lost when updating docs. This commit should
fix it.

Signed-off-by: Sheogorath <1c2647ce2ee8a4ec8e5c8b1b245ca38f94844bab@shivering-isles.com>

Yukaii/hackmd,hackmdio/hackmd,hackmdio/hackmd,Yukaii/hackmd,jackycute/HackMD,jackycute/HackMD,hackmdio/hackmd,Yukaii/hackmd,jackycute/HackMD