docs: rename master to main

Author: nikku

README.md

@@ -35,7 +35,7 @@ Validate your diagrams via the commandline:
 
 ## Rules
 
-Our [documentation](https://github.com/bpmn-io/bpmnlint/tree/master/docs/rules#rules) lists all currenty implemented rules, the [`./rules` folder](https://github.com/bpmn-io/bpmnlint/tree/master/rules) contains each rules implementation.
+Our [documentation](https://github.com/bpmn-io/bpmnlint/tree/main/docs/rules#rules) lists all currenty implemented rules, the [`./rules` folder](https://github.com/bpmn-io/bpmnlint/tree/main/rules) contains each rules implementation.
 
 Do you miss a rule that should be included? [Propose a new rule](https://github.com/bpmn-io/bpmnlint/issues/new?assignees=&labels=rules&template=NEW_RULE.md).