[README] Fix typo function name in README
[README] Fix typo function name in README

This change replaces `getIcon` with `getIconByName` in README.
`getIconByName` function is imported in the example, but `getIcon` is used (`getIcon` is not imported).
So I guess `getIcon` is a typo of `getIconByName`.

primer/octicons,primer/octicons,primer/octicons,primer/octicons