Typo in DEVELOPING guide

[dillfitz]

In the DEVELOPING guide, there is a typo in the example under the heading "Working with documents: metadata"

In particular:

$ docker exec -i -t opendatacernch-web-1 cernopendata fixtures docs \
     --mode insert-or-replace \
     -f data/docs/lhcb-about/lhcb-about.json

Should be updated to:

$ docker exec -i -t opendatacernch-web-1 cernopendata fixtures docs \
     --mode insert-or-replace \
     -f /content/data/docs/lhcb-about/lhcb-about.json

[psaiz]

Thanks for pointing it out!