fix(multi-stage): fix outdated stage name reference

In commit 2a6f7eac80, the Dockerfile under the section "Name your build
stages" was updated, the build stage was renamed from "builder" to
"build". However, the next section was still using the old build stage
name.

Fixes: 2a6f7eac80 ("freshness: update top pages in the build section")
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

build/building/multi-stage.md

@@ -92,7 +92,7 @@ CMD ["/bin/hello"]
 When you build your image, you don't necessarily need to build the entire
 Dockerfile including every stage. You can specify a target build stage. The
 following command assumes you are using the previous `Dockerfile` but stops at
-the stage named `builder`:
+the stage named `build`:
 
 ```console
 $ docker build --target build -t hello .