Monitoring_uptime_check_config headers needs a better description than now

[eternityduck]

What kind of contribution is this issue about?

To make better docs about Monitoring_uptime_check_config.

Links:

https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/monitoring_uptime_check_config
(https://registry.terraform.io/providers/hashicorp/google/latest/docs/resources/monitoring_uptime_check_config#:~:text=The%20list%20of%20headers%20to%20send%20as%20part%20of%20the%20uptime%20check%20request.)

Details

I suggest you to change the description of headers argument reference, cause it's says that The list of headers to send as part of the uptime check request, but actually this argument accepts a map of headers(key:value), so it will be good if you change description to make people understand type of argument better