Uganda - Fix holiday provider mapping (#946)

Author: tinohager

src/Nager.Date/HolidaySystem.cs

@@ -140,6 +140,7 @@ public static class HolidaySystem
                 { CountryCode.TN, new Lazy<IHolidayProvider>(() => new TunisiaHolidayProvider())},
                 { CountryCode.TR, new Lazy<IHolidayProvider>(() => new TurkeyHolidayProvider())},
                 { CountryCode.UA, new Lazy<IHolidayProvider>(() => new UkraineHolidayProvider(_orthodoxProvider))},
+                { CountryCode.UG, new Lazy<IHolidayProvider>(() => new UgandaHolidayProvider(_catholicProvider))},
                 { CountryCode.US, new Lazy<IHolidayProvider>(() => new UnitedStatesHolidayProvider(_catholicProvider))},
                 { CountryCode.UY, new Lazy<IHolidayProvider>(() => new UruguayHolidayProvider(_catholicProvider))},
                 { CountryCode.VA, new Lazy<IHolidayProvider>(() => new VaticanCityHolidayProvider(_catholicProvider))},