Update and fix descriptor buffer tutorial

Description

This PR fixes a type on the tutorial for the descriptor buffer and also updates a code block to be inline with recent changes to that sample.

Note: This is a pure documentation fix

Please ensure the following points are checked:

[ ] My code follows the coding style
[ ] I have reviewed file licenses
[ ] I have commented any added functions (in line with Doxygen)
[ ] I have commented any code that could be hard to understand
[ ] My changes do not add any new compiler warnings
[ ] My changes do not add any new validation layer errors or warnings
[ ] I have used existing framework/helper functions where possible
[ ] My changes do not add any regressions
[ ] I have tested every sample to ensure everything runs correctly
[ ] This PR describes the scope and expected impact of the changes I am making

Note: The Samples CI runs a number of checks including:
- [ ] I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
- [ ] My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions