Fix typo in CONTRIBUTING.md (#3769)

Author: igrlk

.github/CONTRIBUTING.md

@@ -26,7 +26,7 @@ change to the spec, please first open an
 
 ### GraphQL Specification Membership Agreement
 
-This repository is managed by EasyCLA. Project participants must sign the free ([GraphQL Specification Membership agreement](https://preview-spec-membership.graphql.org) before making a contribution. You only need to do this one time, and it can be signed by [individual contributors](http://individual-spec-membership.graphql.org/) or their [employers](http://corporate-spec-membership.graphql.org/).
+This repository is managed by EasyCLA. Project participants must sign the free [GraphQL Specification Membership agreement](https://preview-spec-membership.graphql.org) before making a contribution. You only need to do this one time, and it can be signed by [individual contributors](http://individual-spec-membership.graphql.org/) or their [employers](http://corporate-spec-membership.graphql.org/).
 
 To initiate the signature process please open a PR against this repo. The EasyCLA bot will block the merge if we still need a membership agreement from you.