docs/manual/br2-external: remove superfluous leading space in code blocks

They are not needed, and other code blocks in the same file are not
indented either.

Remove those leading spaces, so all our code blocks look the same.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>