Fix typos

[apetenchea]

While reading the documentation, I stumbled across the following typos:

• xtheadsync/sync_is.adoc: incomplete synopsis
• xtheadmemidx/lbuia.adoc: rs register should be rd in the example
• xtheadmemidx/ldib.adoc: mnemonic shows lwib instead of ldib

[CLAassistant]

All committers have signed the CLA.

[apetenchea]

Thanks @cmuellner for the review! I figured this is already the best place for suggesting another small fix on the ldib mnemonic, so I added another commit: https://github.com/T-head-Semi/thead-extension-spec/pull/53/commits/5db5d28251a90760a9856110d222bcbde1f2f532