Update build.md

There might by a slight error when illustrating the `additional_contexts` attribute. The compose file declares two services with the same name.
It might induce misunderstanding, especially since the second service's image uses the first one to build upon.

content/reference/compose-file/build.md

@@ -130,7 +130,7 @@ services:
     dockerfile_inline: |
       FROM alpine
       RUN ...
- base:
+ my-service:
   build:
     context: .
     dockerfile_inline: |