Rename typescript-checkJS script to typescript-checkJs

For better consistency with the TSConfig option name.

The script had been broken for a long time, so it is safe to assume
nobody relies on the old name in custom scripts.

package.json

@@ -25,7 +25,7 @@
     "test": "jest && npm run test-typings",
     "test-esm": "node ./tests/esm-imports-test.mjs",
     "test-typings": "tsd",
-    "typescript-checkJS": "tsc -p tsconfig.checkJs.json",
+    "typescript-checkJs": "tsc -p tsconfig.checkJs.json",
     "test-all": "npm run test && npm run lint && npm run typescript-checkJS && npm run test-esm"
   },
   "files": [