Remove incorrect duplicate phrase from build.md

[pjaffe]

A phrase from the "Text files" section was incorrectly duplicated under the "Tarball contexts" section.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "patch-1" git@github.com:pjaffe/docker.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[pjaffe]

I established a new pull request #31395 that contained the required signature in the commit message. Apparently this Pull Request was still created, despite the "All checks have failed" upon its creation, so I'm closing it out.