Fixed expansion of "plugins"

Closes #26

There was no need to add @list, and it was causing JSON-LD to expand the
URI of 'plugins'

senpy/schemas/context.jsonld

@@ -37,9 +37,6 @@
       "@type": "@id",
       "@container": "@set"
     },
-    "plugins": {
-      "@container": "@list"
-    },
     "options": {
       "@container": "@set"
     },

senpy/schemas/plugins.json

@@ -10,8 +10,6 @@
           "items": {
             "$ref": "plugin.json"
           }
-        },
-        "@type": {
         }
       }
     }