Fix cwltool:Loop extension definition

This commit fixes two small errors in the `cwltool:Loop` definition that
were breaking the Schema SALAD codegen procedure.

Author: GlassOfWhiskey

cwltool/extensions-v1.2.yml

@@ -159,6 +159,7 @@ $graph:
         default must be applied prior to scattering or evaluating `valueFrom`.
       jsonldPredicate:
         _id: "sld:default"
+        _container: "@list"
         noLinkCheck: true
     valueFrom:
       type:
@@ -236,7 +237,7 @@ $graph:
         name: LoopOutputModes
         symbols: [ last, all ]
       default: last
-      doc:
+      doc: |
         - Specify the desired method of dealing with loop outputs
         - Default. Propagates only the last computed element to the subsequent steps when the loop terminates.
         - Propagates a single array with all output values to the subsequent steps when the loop terminates.