Update package.json

Possible fix for:
```
node:internal/modules/cjs/loader:361
      throw err;
      ^

Error: Cannot find module '/Users/frankbouwens/priv/SOME_PROJECT/node_modules/@2toad/diff-match-patch/diff_match_patch.js'. Please verify that the package.json has a valid "main" entry
```

Author: frankkienl

package.json

@@ -9,7 +9,7 @@
     "diff-match-patch",
     "levenshtein"
   ],
-  "main": "diff_match_patch.js",
+  "main": "diff-match-patch.js",
   "directories": {
     "test": "tests"
   },