Update example in the Compose Build Specification (additional_contexts) to ease it's understanding (#22140)

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.

## Description

I replaced the key defining the second service of the compose file used
to illustrate the `additional_contexts` attribute, since the key `base`
is used twice. Of course, any better replacement could be used, instead
of `my-service`.

It is invalid to use the same key twice (the error: `mapping key "base"
already defined at line 2` is expected on `docker compose up`), and it
does harm the understanding of the illustrated point of building a
service's image using another one to share layers.

## Reviews

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content/reference/compose-file/build.md

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