l3build: fix bug in "doc"; this space is seemingly important

before the fix I was getting:

    ! I can't find file `" "'.
    <to be read again> 
                       \let 
    <*> " \input
                 breqn.dtx"

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5094 de43f980-851b-0410-b2f7-c40aca1f87e0