Fix nginx example (cache immutable)

[CarlSchwan]

Signed-off-by: Carl Schwan carl@carlschwan.eu

[icewind1991]

After some more testing I realized that nginx doesn't use the query params when matching location blocks so that can't be used to distinguish the requests in this way.

I've submitted a version using map here: #8083 which works in my testing (without ?v=1 you get normal cache-control, with ?v=1 you get cache-control: immutable)