Change String to JsonDecodeError to fix docs typo (#88)

* fix: change String to JsonDecodeError in docs

* Add changes to the changelog

* Moved changes to "Unreleased" section in changelog

Author: BrandonHowe

docs/CHANGELOG.md

@@ -10,6 +10,8 @@ New features:
 
 Bugfixes:
 
+* Changed type in docs of `decodeJson` from `forall a. DecodeJson a => Json -> Either String a` to `forall a. DecodeJson a => Json -> Either JsonDecodeError a`.
+
 Other improvements:
 
 ## [0.0.0] - 2020-01-01

docs/README.md

@@ -75,7 +75,7 @@ We can automatically decode `Json` using the `DecodeJson` type class ([pursuit](
 Every type within `User` has an instance for `DecodeJson`, which means we can use the `decodeJson` function to try to decode a `Json` value into our type. Once again, integer and string values will be decoded directly from the `Json`, but containing types like `Record` and `Maybe` will also require instances for the types they contain.
 
 ```purs
-decodeJson :: DecodeJson a => Json -> Either String a
+decodeJson :: DecodeJson a => Json -> Either JsonDecodeError a
 ```
 
 > Tip: To parse a JSON string as a `Json` value, you can use the `parseJson` function (which can fail). If you are sure you have valid JSON, then consider writing it in an FFI file and foreign importing it as `Json` as described in the [`argonaut-core` documentation](https://github.com/purescript-contrib/purescript-argonaut-core#introducing-json-values).