maintainer: update doc file path to accept all under doc/ dir

If we use doc/* maintainer will only take care of files inside
doc/__. Instead if we use plain doc/ maintainer will be responsible
for all the content inside doc directory.

Change-Id: I1b69667a42231d3de826413eb5aa281b7f68d61a
Signed-off-by: Javier B Perez <javier.b.perez.hernandez@intel.com>