RangeCalendar format propType to work with array

[onlyann]

The format prop accepts both string and string[].

Update the format prop type to match the underlying CalendarPart component.

Without this change, passing an array of formats to the RangeCalendar component works but results in a warning.

[codecov[bot]]

Codecov Report

Merging #613 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #613   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files          10       10           
  Lines         716      716           
  Branches      214      214           
=======================================
  Hits          650      650           
  Misses         57       57           
  Partials        9        9

Impacted Files	Coverage Δ
src/RangeCalendar.js	89.8% <ø> (ø)	:arrow_up:

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