Update covid19za_timeline_testing.csv

[camappel]

While performing an audit of our sources for Our World in Data, I noticed a small typo in your database. On 18 April 2020, there had been 108,021 tests conducted, not 108,201, according to the NICD (https://twitter.com/nicd_sa/status/1251595223534362625).

[vukosim]

Thank you so much @camappel