Remove camel case from imageuploadtype in config

This removes the only camel cased option of the config options
**we** added to the config.json.

In auth provider's config parts are a lot of camel cased options
provided. We shouldn't touch them to keep them as similar as
possible to the examples.

Fixes #315

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

docs/guides/minio-image-upload.md

@@ -62,7 +62,7 @@ Minio Guide for HackMD
 8. Set upload type.
 
    ```JSON
-   "imageUploadType": "minio"
+   "imageuploadtype": "minio"
    ```
 
 9. Review your config.
@@ -78,6 +78,6 @@ Minio Guide for HackMD
        "secure": false
      },
      "s3bucket": "hackmd",
-     "imageUploadType": "minio"
+     "imageuploadtype": "minio"
    }
    ```

docs/guides/s3-image-upload.md

@@ -63,7 +63,7 @@
     {
         "production": {
             ...
-            "imageUploadType": "s3",
+            "imageuploadtype": "s3",
             "s3": {
                 "accessKeyId": "YOUR_S3_ACCESS_KEY_ID",
                 "secretAccessKey": "YOUR_S3_ACCESS_KEY",