wrong instruction aboud `downloadAssets` task in README

[Letsric]

Steps to reproduce:

1. Follow the Instructions in the README
2. The following error occurs at the asset downloading step

./gradlew downloadAssets

FAILURE: Build failed with an exception.

* What went wrong:
Task 'downloadAssets' not found in root project 'minecraft-1.21.11' and its subprojects.

This happens, because the README still references the downloadAssets gradle task. This task was renamed to assets in commit bdb813f (see build.gradle, line 267).

Suggested fix:

• Either update the README to refer to the assets task instead, or
• remove the explicit download step, as it is not needed (runClient depends on assets, so running ./gradlew runClient will download assets automatically)