docs(README): correct the default value for [hash:<length>] (#230)

Author: sheerun

README.md

@@ -116,7 +116,7 @@ You can configure a custom filename template for your file using the query param
 |:--:|:--:|:-----:|:----------|
 |**`hashType`**|`{String}`|`md5`|`sha1`, `md5`, `sha256`, `sha512`|
 |**`digestType`**|`{String}`|`base64`|`hex`, `base26`, `base32`, `base36`, `base49`, `base52`, `base58`, `base62`, `base64`|
-|**`length`**|`{Number}`|`8`|The length in chars|
+|**`length`**|`{Number}`|`9999`|The length in chars|
 
 By default, the path and name you specify will output the file in that same directory and will also use that same URL path to access the file.