Fix typo in datetime concept

Author: Bahaaio

concepts/datetime/about.md

@@ -47,7 +47,7 @@ These methods return a _new_ `LocalDate` instance and do not update the existing
 ```java
 LocalDate date = LocalDate.of(2007, 12, 3);
 
-date.addDays(3);
+date.plusDays(3);
 // => 2007-12-06
 
 date.addMonths(1);

concepts/datetime/introduction.md

@@ -41,7 +41,7 @@ These methods return a _new_ `LocalDate` instance and do not update the existing
 ```java
 LocalDate date = LocalDate.of(2007, 12, 3);
 
-date.addDays(3);
+date.plusDays(3);
 // => 2007-12-06
 ```
 

exercises/concept/booking-up-for-beauty/.docs/introduction.md

@@ -43,7 +43,7 @@ These methods return a _new_ `LocalDate` instance and do not update the existing
 ```java
 LocalDate date = LocalDate.of(2007, 12, 3);
 
-date.addDays(3);
+date.plusDays(3);
 // => 2007-12-06
 ```