mbed::File class example doesn't use mbed::File class

# Docs issue template

## Page link
https://os.mbed.com/docs/mbed-os/v6.10/apis/file.html

## Expected to find...
The example should show the use of the mbed::File class. 
It would be best if there were 2 examples; the current example using the stdio functions, plus a new example showing the usage of the mbed::File class.

## Found
The example uses the stdio file functions.
