bugfix: Recurse on FieldKind for NonNull/List Types

Fixes where NonNull and ListTypes were defaulting to scalar types, because we were returning field_type for their kind, which is just that objects name.

Looking at the `schema.json` fixture for Droid (updated test):

```json
{
	"args": [
                {
                  "defaultValue": null,
                  "description": null,
                  "name": "episode",
                  "type": {
                    "kind": "NON_NULL",
                    "name": null,
                    "ofType": {
                      "kind": "SCALAR",
                      "name": "String",
                      "ofType": null
                    }
                  }
                }
              ],
              "deprecationReason": null,
              "description": "Get droids for episode",
              "isDeprecated": false,
              "name": "droidsInEpisode",
              "type": {
                "kind": "LIST",
                "name": null,
                "ofType": {
                  "kind": "OBJECT",
                  "name": "Droid",
                  "ofType": null
                }
              }
            }
```

Previously, the `field_kind` function would return `Droid`. Now, it will properly return `OBJECT` and links will render properly.

lib/graphql_markdown/schema.ex

@@ -64,10 +64,10 @@ defmodule GraphqlMarkdown.Schema do
   def field_kind(type) do
     case type["kind"] do
       "NON_NULL" ->
-        field_type(type["ofType"])
+        field_kind(type["ofType"])
 
       "LIST" ->
-        field_type(type["ofType"])
+        field_kind(type["ofType"])
 
       _ ->
         type["kind"]

test/graphql_markdown_test.exs

@@ -80,7 +80,7 @@ defmodule GraphqlMarkdownTest do
 
       # anchors need to be downcased to match other parts of the generated markdown
       content = File.read!("guides/queries.md")
-      assert content =~ "Type: [Droid](scalars.html#droid)"
+      assert content =~ "Type: [Droid](objects.html#droid)"
 
       # union types need to be a valid type for links
       content = File.read!("guides/mutations.md")