Docs need two hyphens for config.file CLI option

[haysclark]

Existing docs appear to be missing a hyphen, causing a syntax error for users.

docker run -p 9090:9090 -v "$(pwd)/prometheus-data":/prometheus-data prom/prometheus -config.file=/prometheus-data/prometheus.yml

Should be:

docker run -p 9090:9090 -v "$(pwd)/prometheus-data":/prometheus-data prom/prometheus --config.file=/prometheus-data/prometheus.yml