chore: update contributing guide

Author: woothu

CONTRIBUTING.md

@@ -101,11 +101,11 @@ included in the project:
 2. If you cloned a while ago, get the latest changes from upstream:
 
    ```bash
-   git checkout master
-   git pull upstream master
+   git checkout dev
+   git pull upstream dev
    ```
 
-3. Create a new topic branch (off the main project development branch) to
+3. Create a new topic branch (off the development branch "dev") to
    contain your feature, change, or fix:
 
    ```bash
@@ -121,7 +121,7 @@ included in the project:
 5. Locally merge (or rebase) the upstream development branch into your topic branch:
 
    ```bash
-   git pull [--rebase] upstream master
+   git pull [--rebase] upstream dev
    ```
 
 6. Push your topic branch up to your fork: