Documentation: filter-branch: show --ignore-unmatch in main index-filter example

Documentation: filter-branch: show --ignore-unmatch in main index-filter example

Rearrange the example usage of

  git filter-branch --index-filter 'git rm --cached ...'

so that --ignore-unmatch is in the main example block.  People keep
stumbling over the (lack of this) option to the point where it is a
FAQ, so we would want to expose the most common usage where it stands
out.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>