MAINTAINERS: Fix the location of tools manuals

MAINTAINERS: Fix the location of tools manuals

The qemu-img.rst, qemu-nbd.rst, virtfs-proxy-helper.rst, qemu-trace-stap.rst,
and virtiofsd.rst manuals were moved to docs/tools, so this update MAINTAINERS
accordingly.

Fixes: a08b4a9fe6c ("docs: Move tools documentation to tools manual")
Signed-off-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210204135425.1380280-1-wainersm@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>