txnbuild: validate bump sequence number

[howardtw]

What version are you using?

txnbuild v10.0.0. (master)

What did you do?

I checked the code and found out that the BumpSequence validate function incorrectly uses the validateAmount helper to check for the sequence number.

What did you expect to see?

A helper function made for checking the valid range of sequence number.

What did you see instead?

A helper function made for checking the "stellar amount" is used to check the sequence number.