docs: update image on recent blog post (#95)

docs/blog/2021-08-04-test-data.md

@@ -6,7 +6,7 @@ author_url: https://github.com/getsynth
 author_image_url: https://avatars.githubusercontent.com/u/12720758?v=4
 tags: [test data, data generation, best practice, gdpr, pseudonymisation]
 description: Some thoughts on why using prod data for testing is still commonplace and why it is time to change this.
-image: https://i.imgur.com/mErPwqL.png
+image: https://freesvg.org/img/TestScreen_square1.png
 hide_table_of_contents: false
 ---