Fix unaesthetic indentation

The current documentation build looks like this:

$ make htmldocs
  SPHINX  htmldocs --> file:///home/willy/kernel/linux-next/Documentation/output
make[2]: Nothing to be done for 'html'.
WARNING: The kernel documentation build process
        support for Sphinx v3.0 and above is brand new. Be prepared for
        possible issues in the generated output.
        $

That extra indentation before my next prompt isn't pretty.  This patch
fixes it, but I'm not a pythonista, and maybe there's a better way.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20210215161757.GD2858050@casper.infradead.org
[jc: tweaked for the "better way"]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>