Unwrap non-indented line in #generate_preamble

The call to `#result` is a method of the preceeding line but the lack of
indentation doesn't make this apparent. So unwrap it back to the preceding
line. This makes the line longer but not unusually so for the rest of the
file.
Merge pull request #28 from alphagov/unwrap_unindented_line

Unwrap non-indented line in #generate_preamble