docs: Fixed group_variable resource docs

Author: maxwoerner

docs/resources/group_variable.md

@@ -27,7 +27,7 @@ resource "gitlab_group_variable" "example" {
 
 # Example  with masked and hidden
 resource "gitlab_group_variable" "example" {
-  project   = "12345"
+  group     = "12345"
   key       = "group_variable_key"
   value     = "group_variable_value"
   masked    = true