Fix spec-md syntax

Author: benjie

spec/GraphQLOverHTTP.md

@@ -376,7 +376,7 @@ And the body:
 
 # Response
 
-When a server receives a {GraphQL-over-HTTP request}, it must return a
+When a server receives a _GraphQL-over-HTTP request_, it must return a
 well‐formed response. The server's response describes the result of validating
 and executing the requested operation if successful, and describes any errors
 encountered during the request.