Replace master/slave terminology in Cluster example

This terminology is frowned upon within my firm and I accidentally forwarded this example to 750 staff.

docs/guides/cluster.md

@@ -22,11 +22,11 @@ with Diagram("Simple Web Service with DB Cluster", show=False):
     web = ECS("service")
 
     with Cluster("DB Cluster"):
-        db_master = RDS("master")
+        db_primary = RDS("primary")
-        db_master - [RDS("slave1"),
+        db_primary - [RDS("replica1"),
-                     RDS("slave2")]
+                     RDS("replica2")]
 
-    dns >> web >> db_master
+    dns >> web >> db_primary
 ```
 
 ![simple web service with db cluster diagram](/img/simple_web_service_with_db_cluster_diagram.png)
@@ -68,4 +68,4 @@ with Diagram("Event Processing", show=False):
 
 ![event processing diagram](/img/event_processing_diagram.png)
 
-> There is no depth limit of nesting. Feel free to create nested clusters as deep as you want.
+> There is no depth limit of nesting. Feel free to create nested clusters as deep as you want.