update readme for TS @types/node-fetch

[adamellsworth]

Purpose

Closes #1396

Changes

Updates Readme such that the @types/node-fetch refs the last known version with valid types (@types/node-fetch@2.x); targets users who cannot upgrade to node-fetch@3.x

Additional information

n/a

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• I updated readme

Note to reviewers of this PR:
because of the minuscule nature of this change I have not updated the Changelog – I feel such an addition would add a bit of unnecessary "noise" to the real additions to the codebase.
also, ty for writing such an amazing lib <3

[LinusU]

👍