Update and rename FIRST-TIMERS.md to FIRST_TIMERS.md

Author: childish-sambino

FIRST-TIMERS.md renamed to FIRST_TIMERS.md

@@ -37,15 +37,15 @@
  6. __Locally merge (or rebase)__ the upstream development branch into your topic branch:
 
    ```    
-    git pull [--rebase] upstream master
+    git pull [--rebase] upstream main
    ``` 
 
  7. __Push__ your topic branch up to your fork:
    ```    
    git push origin <topic-branch-name>
    ```
  8. __[Open a Pull Request](https://help.github.com/articles/creating-a-pull-request/#changing-the-branch-range-and-destination-repository/)__
-    with a clear title and description against the `master` branch. All tests must be passing before we will review the PR.
+    with a clear title and description against the `main` branch. All tests must be passing before we will review the PR.
 
  ### Important notice
  Before creating a pull request, make sure that you respect the repository's constraints regarding contributions. You can find them in the [CONTRIBUTING.md](./CONTRIBUTING.md) file.
@@ -74,4 +74,4 @@
 * [Open Source Data Collector](https://github.com/sendgrid/open-source-library-data-collector/issues?utf8=%E2%9C%93&q=is%3Aopen+label%3A%22difficulty%3A+easy%22+label%3A%22status%3A+help+wanted%22)
 * [Open API Definition](https://github.com/sendgrid/sendgrid-oai/issues?utf8=%E2%9C%93&q=is%3Aopen+label%3A%22difficulty%3A+easy%22+label%3A%22status%3A+help+wanted%22)
 * [DX Automator](https://github.com/sendgrid/dx-automator/issues?utf8=%E2%9C%93&q=is%3Aopen+label%3A%22difficulty%3A+easy%22+label%3A%22status%3A+help+wanted%22)
-* [Documentation](https://github.com/sendgrid/docs/issues?utf8=%E2%9C%93&q=is%3Aopen+label%3A%22difficulty%3A+easy%22+label%3A%22status%3A+help+wanted%22)
+* [Documentation](https://github.com/sendgrid/docs/issues?utf8=%E2%9C%93&q=is%3Aopen+label%3A%22difficulty%3A+easy%22+label%3A%22status%3A+help+wanted%22)