Add missing 'in' in SSO specification

Signed-off-by: Johannes Marbach <n0-0ne+github@mailbox.org>

changelogs/client_server/newsfragments/1748.clarification

@@ -0,0 +1 @@
+Add missing 'in' in SSO specification for grammatical correctness

content/client-server-api/modules/sso_login.md

@@ -10,7 +10,7 @@ This module allows a Matrix homeserver to delegate user authentication
 to an external authentication server supporting one of these protocols.
 In this process, there are three systems involved:
 
--   A Matrix client, using the APIs defined this specification, which
+-   A Matrix client, using the APIs defined in this specification, which
     is seeking to authenticate a user to a Matrix homeserver.
 -   A Matrix homeserver, implementing the APIs defined in this
     specification, but which is delegating user authentication to the