Mention ./promtool check config prometheus.yml needed

[Ricky-Tigg]

Hello. Observations that apply to the whole documentation at prometheus.io:

• By default the query in web browser for http://localhost:9090 opens at the Graph-named tab; which URL does imply http://localhost:9090/graph. Then the mention graph as part of the location path is unneeded.
• Causing a modification to the Prometheus server's configuration file -prometheus.yml- does not come along with a recommendation of verification of the validity of its syntax. Knowing that at http://localhost:9090/config, no such verification is operated, this validation is left undone upon the current documentation. It would be worth rectifying this lack of instruction by including as a good practise such a verification, thus mentioning the related command ./promtool check config prometheus.yml. Illustration:

$ ./promtool check config prometheus.yml
Checking prometheus.yml
 SUCCESS: prometheus.yml is valid prometheus config file syntax