update ContentNotAcceptable error description (#4212)

The `ContentNotAcceptableError` is triggered when client either requests
an unsupported media type, or has form errors such as sending multiples.
Updating the description to also indicate non-supported Accept headers.

Author: etan-status

beacon_chain/rpc/rest_constants.nim

@@ -151,7 +151,7 @@ const
   InvalidLogLevelValueError* =
     "Invalid log level value error"
   ContentNotAcceptableError* =
-    "Could not find out accepted content type"
+    "Accepted media type not supported"
   InvalidAcceptError* =
     "Incorrect accept response type"
   MissingSubCommitteeIndexValueError* =