4c9f04bb added the build directory, so drop the docs for `mkdir`.

Change-Id: Idc7437c0c72666f2c115bb0ddbd753a3179dba79
Reviewed-on: https://swiftshader-review.googlesource.com/c/24508Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>