Correct `isValid` explanation

subjects/date-is/README.md

@@ -4,11 +4,11 @@
 
 Create the following functions:
 
-- `isValid`, this function must return false if its an Invalid Date
+- `isValid`, this function must return false if it's an Invalid Date
 - `isAfter`, this function will receive two valid dates and return true if the first date is bigger then the second date
 - `isBefore`, this function will receive two valid dates and return true if the first date is lesser then the second date
-- `isFuture`, will return true if it is a valid the date given as parameter is higher then the present date
-- `isPast`, will return true if it is a valid date given as parameter is less than the present date
+- `isFuture`, will return true if the date given as parameter is valid and higher than the present date
+- `isPast`, will return true if the date given as parameter is valid and less than the present date
 
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