Pin version of Prettier

Per the Prettier docs recommendation.

Author: wimpyprogrammer

package.json

@@ -61,7 +61,7 @@
     "jest": "^26.0.0",
     "jest-when": "^3.1.0",
     "npm-run-all": "^4.1.5",
-    "prettier": "^2.2.1",
+    "prettier": "2.2.1",
     "pretty-quick": "^3.1.0",
     "regex-to-strings": "^2.0.1",
     "rimraf": "^3.0.2",