chore: links to files in master branch in the README.md and CONTRIBUTING.md

Author: chilango74

CONTRIBUTING.md

@@ -4,7 +4,7 @@ Thank you for your interest in _okama_ and wanting to contribute. Your help is m
 
 Here are some guidelines for contributing.
 
-Please refer to the [RoadMap](https://github.com/mbk-dev/okama/blob/dev/README.md#roadmap) for a list of areas where _okama_ could benefit
+Please refer to the [RoadMap](https://github.com/mbk-dev/okama/blob/master/README.md#roadmap) for a list of areas where _okama_ could benefit
 from. In addition, the following is always welcome:
 
 - Report bugs using [Issues](https://github.com/mbk-dev/okama/issues)

README.md

@@ -171,7 +171,7 @@ The plan for _okama_ is to add more functions that will be useful to investors a
 
 ## Contributing to okama
 
-Contributions are *most welcome*. Have a look at the [Contribution Guide](https://github.com/mbk-dev/okama/blob/dev/CONTRIBUTING.md) for more.
+Contributions are *most welcome*. Have a look at the [Contribution Guide](https://github.com/mbk-dev/okama/blob/master/CONTRIBUTING.md) for more.
 
 ## Communication