Some fixes to "maintain-guides-async-backing.md"

[mrcnski]

Addresses some issues found in paritytech/polkadot-sdk#2826.

[DrW3RK]

Thank you!

[evilrobot-01]

Seems that this PR broke the screenshot image for implementing the AuraUnincludedSegmentApi within a runtime (see Phase 1, Step 7, substep 3 at https://wiki.polkadot.network/docs/maintain-guides-async-backing#phase-1---update-parachain-runtime) due to using the wrong image name.

The replaced image aligned with the config at https://github.com/paritytech/polkadot-sdk/blob/c01dbebeaa6394691974de46dd2d41a582f6a4c2/cumulus/parachains/runtimes/testing/rococo-parachain/src/lib.rs#L839

Edit: note also that the mention of async backing already being part of the cumulus parachain template is incorrect. The link points to a branch on the old cumulus repo and the cumulus template within the Polkadot SDK repo does not yet seem to be async backing ready.

[mrcnski]

You're right, should have created a new image called async-backing-unincluded-segment-capacity.png. Sorry, I missed that the same path was used in two places. Will fix it soon.

Edit: note also that the mention of async backing already being part of the cumulus parachain template is incorrect.

Where is this mention? Let me know what and how to fix and I'll include it in the follow-up PR.

[evilrobot-01]

Hmmm, it seems there is indeed some async backing stuff in https://github.com/paritytech/polkadot-sdk/tree/master/cumulus/parachain-template, so I guess https://github.com/mrcnski/polkadot-wiki/blob/b24edec5c35be36afa772f97ccb9a1530d21812c/docs/maintain/maintain-guides-async-backing.md?plain=1#L16C3-L16C3 just needs updating to point to that location.