Fix documentation for getRawTransaction

[christroutner]

There is discrepancies between documentation and implementation for the getRawTransaction endpoint:

https://github.com/Bitcoin-com/bitbox-javascript-sdk/blob/master/src/RawTransactions.js#L52

Some comments I've received:

The developer docs say that getRawTransaction() array of string will be returned. but actually it is just a single string.

Your swagger page shows that getTransaction() returns a whole object. but the typings say it returns just a string. and then the developer docs say an array of strings. 3 different things!

[christroutner]

Verified that getRawTransaction() returns a string when a single entry is input, and returns an array of strings when an array is input. I think this pattern is not quite so clear when someone is reading the documentation for the first time.

Closing this issue, but will keep an eye out for documentation discrepencies.