Update the patternfly-icons FilterIcon to use the fa-filter rather than pf-filter-icon

[nicolethoen]

Seems that the Filters demo uses the incorrect filter icon:

It should look like:

And the icon docs explain the following about filter icons:

[evwilkin]

FYI V4 does show both the old custom PF filter icon as well as the correct FA filter icon:
https://patternfly-react.surge.sh/icons/

V5 only shows the old custom PF filter icon - the FA filter icon is missing:
http://patternfly-react-v5.surge.sh/icons

This change occurred between 5.0.0-alpha.2 and 5.0.0-alpha.3