Update custom-connectors.mdx

[arunjose696]

Fixing 2 small bugs in docs

Proposed changes:

The tutorial for custom connectors have a few bugs 1)The name function in custom connector should have an argument the self class as the name method of Input class has an argument. This was mentioned in this community post https://forum.rasa.com/t/typeerror-name-takes-0-positional-arguments-but-1-was-given-r/42399/5 2)The curl command to test out the API, sends message in a key "message", but the given code sample for custom connector checks for a key "text", so setting the message to none. This would cause some confusion for users first trying custom connector.

• ...

Status (please check what you already did):

• [ ] added some tests for the functionality
• [ ] updated the documentation
• [ ] updated the changelog (please check changelog for instructions)
• [ ] reformat files using black (please check Readme for instructions)

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