doc(instance): Fix type in security_group doc (#536)

Fix typo in scaleway_instance_security_group documentation title.

Author: Quentin Brosse

website/docs/d/instance_security_group.html.markdown

@@ -5,7 +5,7 @@ description: |-
   Gets information about a Security Group.
 ---
 
-# scaleway_security_group
+# scaleway_instance_security_group
 
 Gets information about a Security Group.