babel-jest: Suggest the correct bridge version

[rstacruz]

Summary

This makes babel-jest's docs congruent with what babel-core suggests. From [email protected] docs:

to install Babel 6, they could now do

`npm i some-package babel-core@^7.0.0-bridge @babel/core

Test plan`

Nothing to test, this is just a documentation change.

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[SimenB]

Oh, missed a spot in #6745, thanks!

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