button style to circle

[tomrule007]

https://github.com/tomrule007/grinding/blob/1b1e3560adb804813f812e628d9223d99830d537/src/App.js#L40-L46

change from component to < Fab /> component reference: https://material-ui.com/components/buttons/#floating-action-buttons

**Really like the Zooming in/out of view animation.