* Docs: Added $__rate_interval to Prometheus help

The variable '$__rate_interval' was added in Grafana 7.2.0. This commit
adds the missing help information to Grafana's Prometheus PromQueryEditor.

Signed-off-by: David Calvert <davidcalvertfr@gmail.com>

* Docs: Fixed line length in PromQueryEditor.tsx

This commit will fix line length of my previous commit to make CI green
again.

Signed-off-by: David Calvert <davidcalvertfr@gmail.com>

* Docs: Fixed ci error

Fixed CI error due to my previous commits

Signed-off-by: David Calvert <davidcalvertfr@gmail.com>