Docs update - fix invalid class name

Author: mpraglowski

ruby_event_store/lib/ruby_event_store/client.rb

@@ -137,7 +137,7 @@ def streams_of(event_id)
     # @param event_id [String]
     # @param stream_name [String]
     # @return [Integer] nonnegative integer position of event in stream
-    # @raise [EventNotInStream]
+    # @raise [EventNotFoundInStream]
     def position_in_stream(event_id, stream_name)
       @repository.position_in_stream(event_id, Stream.new(stream_name))
     end