Fix old references to master in docs

docs/content/doc/development/development.md

@@ -15,7 +15,7 @@ menu:
 We use go modules to vendor libraries for Vikunja, so you'll need at least go `1.11` to use these.
 If you don't intend to add new dependencies, go `1.9` and above should be fine.
 
-To contribute to Vikunja, fork the project and work on the master branch.
+To contribute to Vikunja, fork the project and work on the main branch.
 
 A lot of developing tasks are automated using a Magefile, so make sure to [take a look at it]({{< ref "mage.md">}}).
 

docs/content/doc/development/mage.md

@@ -39,7 +39,7 @@ There are multiple categories of subcommands in the magefile:
 
 ## CI
 
-These tasks are automatically run in our CI every time someone pushes to master or you update a pull request:
+These tasks are automatically run in our CI every time someone pushes to main or you update a pull request:
 
 * `mage check:lint`
 * `mage check:fmt`

docs/content/doc/development/notifications.md

@@ -110,4 +110,4 @@ If it was called, no mails are being sent and you can instead assert they have b
 
 ## Example
 
-Take a look at the [pkg/user/notifications.go](https://code.vikunja.io/api/src/branch/master/pkg/user/notifications.go) file for a good example.
+Take a look at the [pkg/user/notifications.go](https://code.vikunja.io/api/src/branch/main/pkg/user/notifications.go) file for a good example.