[3.8] bpo-41824: Fix indentation issue in ForwardRef docs

[Fidget-Spinner]

Introduced by c8a48c6.

https://bugs.python.org/issue41824

[Fidget-Spinner]

@gvanrossum I'm really sorry, I didn't notice that I didn't indent the rST directive with 3 spaces. On sphinx 3 this causes the docs to render weirdly.

Edit: The 3.9 branch isn't affected, neither is the 3.10/main one.

[miss-islington]

Thanks @Fidget-Spinner for the PR, and @gvanrossum for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7, 3.8, 3.9.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-bot]

@gvanrossum: Please replace # with GH- in the commit message next time. Thanks!

[miss-islington]

Sorry, @Fidget-Spinner and @gvanrossum, I could not cleanly backport this to 3.9 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 822f7c266f886677047338e8b167ba39a6abd91e 3.9

[miss-islington]

Sorry @Fidget-Spinner and @gvanrossum, I had trouble checking out the 3.8 backport branch.
Please backport using cherry_picker on command line.
cherry_picker 822f7c266f886677047338e8b167ba39a6abd91e 3.8

[miss-islington]

Sorry, @Fidget-Spinner and @gvanrossum, I could not cleanly backport this to 3.7 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 822f7c266f886677047338e8b167ba39a6abd91e 3.7

[Fidget-Spinner]

This doesn't need backporting - it only affected the 3.8 branch :). Thanks for your help Guido!