manageBuyOffer inconsistent parameter naming

[MisterTicot]

In js-stellar-base, manageBuyOffer amount parameter has been named buyAmount.

However, in the stellar.org guide it is referenced as amount. Horizon uses amount parameter as well when returning the operation record.

Hence, I assume js-stellar-base should use amount instead of buyAmount for a matter of consistency, which is also more consistent with manageSellOffer & passiveSellOffer amount parameters.

[MisterTicot]

The sooner this one is dealt with, the better: the last thing we want is inconsistent naming to accumulate over time.