Light editorial around delivery agnostic subscriptions (#959)

The phrase "Subscriptions specifies" here was confusing, it's the GraphQL spec that does the specification, so I have clarified what is specified.

Co-authored-by: Roman Ivantsov <[email protected]>

Author: benjie

spec/Section 6 -- Execution.md

@@ -236,12 +236,12 @@ connectivity.
 **Delivery Agnostic**
 
 GraphQL subscriptions do not require any specific serialization format or
-transport mechanism. Subscriptions specifies algorithms for the creation of a
-stream, the content of each payload on that stream, and the closing of that
-stream. There are intentionally no specifications for message acknowledgement,
-buffering, resend requests, or any other quality of service (QoS) details.
-Message serialization, transport mechanisms, and quality of service details
-should be chosen by the implementing service.
+transport mechanism. GraphQL specifies algorithms for the creation of the
+response stream, the content of each payload on that stream, and the closing of
+that stream. There are intentionally no specifications for message
+acknowledgement, buffering, resend requests, or any other quality of service
+(QoS) details. Message serialization, transport mechanisms, and quality of
+service details should be chosen by the implementing service.
 
 #### Source Stream