[Docs] Broken "Exceptions" link in docs/guide/client.md

[SimonAllen0901]

Hi 😃
While reading the documentation, I noticed that the Exceptions link in docs/guide/client.md is broken.

[Yang-33]

Thank you for reporting. It should be https://github.com/line/line-bot-sdk-nodejs/blob/master/lib/exceptions.ts.

[Yang-33]

or (probably this is better)

HTTPError
HTTPFetchError
JSONParseError
ReadError
RequestError
SignatureValidationFailed