Change disabledDates to disabled-dates in readme

[DuckThom]

While originally marked as fixed in #521, the correct syntax is kebab-case instead of camelCase in HTML attributes.

Using camelCase will make Vue show the following message in the console and the dates wont be disabled:

[Vue tip]: Prop "disableddates" is passed to component <Anonymous>, but the declared prop name is "disabledDates". Note that HTML attributes are case-insensitive and camelCased props need to use their kebab-case equivalents when using in-DOM templates. You should probably use "disabled-dates" instead of "disabledDates".

[coveralls]

Coverage remained the same at 100.0% when pulling 1b61cdb on DuckThom:patch-1 into bf28bca on charliekassel:master.