docs: update part5b heading to match React 19 content

Removed "proptypes" from the Part 5b heading as the library is no
longer covered in the React 19 version of the course.

Author: jbdemme

src/content/partnavigation/partnavigation.js

@@ -99,7 +99,7 @@ module.exports = {
     },
     5: {
       a: 'Login in frontend',
-      b: 'props.children and proptypes',
+      b: 'props.children and component refs',
       c: 'Testing React apps',
       d: 'End to end testing: Playwright',
       e: 'End to end testing: Cypress',