Fixed resource references

Author: 7h3Rabbit

README.md

@@ -4,7 +4,7 @@
 
 The latest development code for the CSS Validator is available through [the GitHub repository](https://github.com/w3c/css-validator).
 
-For a visual representation, see the [CSS Validator Sequence Diagram](https://github.com/w3c/css-validator/raw/master/docs/CSS_Validator_Sequence_Diagram.png), which is available in [png](https://github.com/w3c/css-validator/raw/master/docs/CSS_Validator_Sequence_Diagram.png), [graffle](https://github.com/w3c/css-validator/raw/master/docs/CSS_Validator_Sequence_Diagram.graffle) and [svg](https://github.com/w3c/css-validator/raw/master/docs/CSS_Validator_Sequence_Diagram.svg) formats.
+For a visual representation, see the [CSS Validator Sequence Diagram](https://github.com/w3c/css-validator/blob/main/docs/CSS_Validator_Sequence_Diagram.png?raw=true), which is available in [png](https://github.com/w3c/css-validator/blob/main/docs/CSS_Validator_Sequence_Diagram.png?raw=true), [graffle](https://github.com/w3c/css-validator/blob/main/docs/CSS_Validator_Sequence_Diagram.graffle?raw=true) and [svg](https://github.com/w3c/css-validator/blob/main/docs/CSS_Validator_Sequence_Diagram.svg?raw=true) formats.
 
 If you have any questions or problems with the validator, send us an [email](https://github.com/w3c/css-validator/blob/master/Email.html.en).