Update CONTRIBUTING.md to refelect main branch

Signed-off-by: Ferran Rodenas <rodenasf@vmware.com>

CONTRIBUTING.md

@@ -16,19 +16,19 @@ Example:
 
 ``` shell
 git remote add upstream https://github.com/vmware/terraform-provider-vra.git
-git checkout -b my-new-feature master
-git commit -a
+git checkout -b my-new-feature main
+git commit -s -a
 git push origin my-new-feature
 ```
 
 ### Staying In Sync With Upstream
 
-When your branch gets out of sync with the vmware/master branch, use the following to update:
+When your branch gets out of sync with the vmware/main branch, use the following to update:
 
 ``` shell
 git checkout my-new-feature
 git fetch -a
-git pull --rebase upstream master
+git pull --rebase upstream main
 git push --force-with-lease origin my-new-feature
 ```
 
@@ -51,7 +51,7 @@ If you need to squash changes into an earlier commit, you can use:
 ``` shell
 git add .
 git commit --fixup <commit>
-git rebase -i --autosquash master
+git rebase -i --autosquash main
 git push --force-with-lease origin my-new-feature
 ```