[encrypted-media] Correct object name at the beginning of section 6.3 MediaKeyStatusMap

jdsmith3000 has just created a new issue for 
https://github.com/w3c/encrypted-media:

== Correct object name at the beginning of section 6.3 
MediaKeyStatusMap ==
'MediaKeySession' in the sentence below should be 'MediaKeyStatusMap':

6.3 MediaKeyStatusMap Interface

The MediaKeySession object is a read-only map of key IDs to the 
current status of the associated key.


See https://github.com/w3c/encrypted-media/issues/77