React-icons - update README reference of v5 to v6

[mcoker]

Small thing I noticed in the README, the reference to .pf-v5-svg should update to .pf-v6-svg

patternfly-react/packages/react-icons/README.md

Line 17 in 2b5a598

All icons from @patternfly/react-icons have the HTML class pf-v5-svg applied and CSS styles applied via @patternfly/react-styles.

[evwilkin]

Closed by #10575