docs: Fix import command syntax for GitHub custom property documentation (#2871)

Author: deiga

website/docs/r/repository_custom_property.html.markdown

@@ -43,5 +43,5 @@ The following arguments are supported:
 GitHub Repository Custom Property can be imported using an ID made up of a comibnation of the names of the organization, repository, custom property separated by a `:` character, e.g.
 
 ```
-$ terraform import github_repository_custom_property.example <organization-name>:<repo-name>:<custom-property-name>
+$ terraform import github_repository_custom_property.example organization-name:repo-name:custom-property-name
 ```