Add backticks around environment variable names

[dustindortch]

The provider documentation doesn't highlight the environment variables for the provider block. Added backticks to bring attention to the option which is standard within most providers.

[danischm]

Thank you for your contribution! The provider.go is generated from this (https://github.com/CiscoDevNet/terraform-provider-sdwan/blob/main/gen/templates/provider.go) template. Please update the template and then run go generate. Thanks again!