docs: dev appview must be pointed at local knotmirror (port 7007)

while following Hacking on Tangled

after nix run --impure .#vm and pushing to the dev knot. loading the repo page fails with

ERRO appview/repo: ... calling knotmirror git.listBranches ... 
https://mirror.tangled.network/... context canceled

appview/config/config.go:50 defaults TANGLED_KNOTMIRROR_URL to the public mirror nothing tells us to point it to the local knotmirrot on port 7007

i set export TANGLED_KNOTMIRROR_URL=http://localhost:7007 before running appview and it worked