update(docs): postgres.md so the instructions are fit for Spotify as a provider.

[MatthewJamesBoyle]

A typical Spotify avatar URL is as follow:

https://scontent-ams4-1.xx.fbcdn.net/v/t1.0-1/c124.0.320.320a/p320x320/44594471_10217288278180083_8889099770203734016_o.jpg?_nc_cat=103&ccb=2&_nc_sid=0c64ff&_nc_ohc=wHUMWDX3V20AX-k8dIO&_nc_ht=scontent-ams4-1.xx&tp=27&oh=c91bd3a69375dfcc6811a75dcb5c3234&oe=5FD3FCAA

This is 264 characters long and so this db schema does not work and you get an error. Upping the character count for image URL solves the problem.

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[ndom91]

@MatthewJamesBoyle did you also modify this in your db locally?

If so, maybe add the length: 300 option to the actual model definition here as well: https://github.com/nextauthjs/next-auth/blob/canary/src/adapters/typeorm/models/user.js

In which case, do you mind modifying the documention for the schema for mysql and mssql as well (same folder as your modified postgres.md file)?

Thanks!

EDIT: Theres another PRs open addressing this same issue (#777) - the going suggestion is to change the field from varchar to text

[balazsorban44]

Fixed through #777