Fix grammatical error in name validation error

[weiseng18]

Closes #256.

This is allowed in v1.6 as stated in Week 11 Project tab:

[codecov[bot]]

Codecov Report

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

Files with missing lines	Coverage Δ	Complexity Δ
src/main/java/seedu/address/model/person/Name.java	92.85% <ø> (ø)	7.00 <0.00> (ø)