"CSS" instead of "css" in flowchart.md (#4797)

Author: jakeboone02

packages/mermaid/src/docs/syntax/flowchart.md

@@ -709,9 +709,9 @@ flowchart LR
     classDef foobar stroke:#00f
 ```
 
-### Css classes
+### CSS classes
 
-It is also possible to predefine classes in css styles that can be applied from the graph definition as in the example
+It is also possible to predefine classes in CSS styles that can be applied from the graph definition as in the example
 below:
 
 **Example style**