encoding/openapi: correct typo

Change-Id: I864b86c333069e7e1cacf02ed9d140e13c29ad2a
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2461
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/openapi.go b/encoding/openapi/openapi.go
index 74cf693..d481b91 100644
--- a/encoding/openapi/openapi.go
+++ b/encoding/openapi/openapi.go
@@ -82,7 +82,7 @@
 	}
 
 	schemas := &OrderedMap{}
-	schemas.Set("schema", all)
+	schemas.Set("schemas", all)
 
 	top := &OrderedMap{}
 	top.Set("openapi", "3.0.0")