Fix typo in aapt2 CLI --help

Hi, just noticed this difference while reading through docs and then invoking `aapt2 --help`

The updated spelling matches the public documentation for this CLI switch:
https://developer.android.com/tools/aapt2

Change-Id: I37788ab78d7ec6df409b3aa2e1e365626a47ab72
Flag: EXEMPT log only update