Fix typo in serializers.md (#1307)

In the document, ISO 6801 is modified to ISO 8601.
The date and time format standard is ISO 8601, not ISO 6801.

Author: wplong11

docs/serializers.md

@@ -815,7 +815,7 @@ All the previous approaches to specifying custom serialization strategies were _
 fully defined at compile-time. The exception was the [Passing a serializer manually](#passing-a-serializer-manually)
 approach, but it worked only on a top-level object. You might need to change the serialization
 strategy for objects deep in the serialized object tree at run-time, with the strategy being selected in a context-dependent way.
-For example, you might want to represent `java.util.Date` in JSON format as an ISO 6801 string or as a long integer
+For example, you might want to represent `java.util.Date` in JSON format as an ISO 8601 string or as a long integer
 depending on a version of a protocol you are serializing data for. This is called _contextual_ serialization, and it
 is supported by a built-in [ContextualSerializer] class. Usually we don't have to use this serializer class explicitly—there 
 is the [Contextual] annotation providing a shortcut to