Sample code for disbursement in README file.

[arthurarty]

Is your feature request related to a problem? Please describe.
The README section talking about how to use the Disbursements imports the Disbursement class from the mtnmomo.collection however this class is actually found in mtnmomo.disbursement.

Describe the solution you'd like
I would like to correct the two code snippets that have this error in the README.