Fix link to CONTRIBUTING.md

We use "main" branch since 13365c9

Author: tricknotes

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,5 @@
 Hi! Thanks for creating a pull request. 
 
-Please make sure you have read the [Contributing](https://github.com/grahamearley/FirestoreGoogleAppsScript/blob/master/CONTRIBUTING.md) guidelines before submitting. 
+Please make sure you have read the [Contributing](https://github.com/grahamearley/FirestoreGoogleAppsScript/blob/main/.github/CONTRIBUTING.md) guidelines before submitting. 
 
 (You can delete this text).

.github/README.md

@@ -150,7 +150,7 @@ See other library methods and details [in the wiki](/grahamearley/FirestoreGoogl
   > Utilize `createDocument(path + '/' + documentId, fields)` instead to create a document with a specific ID. 
 
 ## Contributions
-Contributions are welcome — send a pull request! See [here](/grahamearley/FirestoreGoogleAppsScript/blob/master/.github/CONTRIBUTING.md) for more information on contributing.
+Contributions are welcome — send a pull request! See [here](/grahamearley/FirestoreGoogleAppsScript/blob/main/.github/CONTRIBUTING.md) for more information on contributing.
 
 After cloning this repository, you can push it to your own private copy of this Google Apps Script project to test it yourself. See [here](https://github.com/google/clasp) for directions on using `clasp` to develop App Scripts locally.
 Install all packages from `package.json` with a bare `npm install`.