Fix SampleDataUtil having same Nric

[yong-zhuo]

Closes #61

Fix duplicate NRIC for sample person data causing duplicate person errors

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ	Complexity Δ
.../java/seedu/address/model/util/SampleDataUtil.java	20.00% <ø> (ø)	1.00 <0.00> (ø)