add explicit prettier-plugin-solidity import to yarn prettier script

Author: ItsNickBarry

package.json

@@ -7,7 +7,7 @@
     "lerna-publish": "yarn build && lerna publish && lerna run tsc-clean",
     "build": "hardhat compile && lerna run tsc",
     "prepare": "husky install",
-    "prettier": "prettier --write .",
+    "prettier": "prettier --write --plugin=prettier-plugin-solidity .",
     "upgrade-dependencies": "yarn-up -e '@solidstate/library,@solidstate/spec,@solidstate/typechain-types' && yarn upgrade"
   },
   "workspaces": [