Clarify service networks documentation

When jumping straight to this bit of the docs, it's not clear
that these are options under a service rather than the top-level
`networks` key. Added a service to make this super clear.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

docs/compose-file.md

@@ -502,6 +502,8 @@ the special form `service:[service name]`.
 Networks to join, referencing entries under the
 [top-level `networks` key](#network-configuration-reference).
 
+    services:
+      some-service:
         networks:
          - some-network
          - other-network
@@ -516,6 +518,8 @@ Since `aliases` is network-scoped, the same service can have different aliases o
 
 The general format is shown here.
 
+    services:
+      some-service:
         networks:
           some-network:
             aliases: