Lexicons, remove invalid concrete schema

lexicons/my/spoor/log/activity.json

@@ -15,7 +15,7 @@
           "progress": {
             "type": "union",
             "refs": [
-              "my.spoor.log.activity#indexProgress"
+              "#progressIndex"
             ]
           },
           "performedAt": {
@@ -31,9 +31,13 @@
         }
       }
     },
-    "indexProgress": {
-      "type": "integer",
-      "description": "The index of the content consumed. Content must be indexable"
+    "progressIndex": {
+      "type": "object",
+      "description": "The index of the content consumed. Content must be indexable",
+      "required": [ "index" ],
+      "properties": {
+        "index": { "type": "integer" }
+      }
     }
   }
 }