fix markdown link in contributing docs (#3320)
fix markdown link in contributing docs (#3320)

the extra `]` is causing the link to render incorrectly
ubschmidt2/terraform-provider-google,terraform-providers/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google,terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,hashicorp/terraform-provider-google,terraform-providers/terraform-provider-google,ubschmidt2/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google,terraform-providers/terraform-provider-google,hashicorp/terraform-provider-google