Fix typo

src/exception.cr

@@ -4,7 +4,7 @@ CallStack.skip(__FILE__)
 
 # Represents errors that occur during application execution.
 #
-# Exception and it's descendants are used to communicate between raise and
+# Exception and its descendants are used to communicate between raise and
 # rescue statements in `begin ... end` blocks.
 # Exception objects carry information about the exception – its type (the
 # exception’s class name), an optional descriptive string, and