Fix incorrect PropType specification
Fix incorrect PropType specification

`PropTypes.function` is incorrect. It should be `PropTypes.func`.

Also a `CustomCounter` without a `count` function doesn't really make sense. So I've marked the `countFunction` as being required.
koaninc/draft-js-plugins,nikgraf/draft-js-plugin-editor,koaninc/draft-js-plugins,dagopert/draft-js-plugins,nikgraf/draft-js-plugin-editor,draft-js-plugins/draft-js-plugins,dagopert/draft-js-plugins,draft-js-plugins/draft-js-plugins,draft-js-plugins/draft-js-plugins,dagopert/draft-js-plugins,nikgraf/draft-js-plugin-editor,draft-js-plugins/draft-js-plugins