fs_config: correct README

Testing on earlier versions of Python indicated an error in the
README. Python's ConfigParser prior to version 3.2 allows for
sections within the same file to override later sections.
Correct the README.

Test: Checked diff and hash of output file from before and after.
Change-Id: Ifd3f78414b4803e4b74decb1fdf6816d4902c4a2
Signed-off-by: William Roberts <william.c.roberts@intel.com>