[Mbed-OS 5.11 OoB] documentation typo

### Description
In the readme, it writes:
For example, in order to compile using the ARM GCC compiler and a K64F target platform use: mbed compile -m K64F -t ARM
However, -t ARM is for ARMCC, for GCC it should be GCC_ARM.

### Issue request type

[ ] Question
[x] Enhancement
[ ] Bug

Open the repository README and inspect the mbed compile example around the compiler option. Done means the example uses the correct compiler identifier for each named compiler; verify the surrounding command formatting and documentation context before updating the typo.

Written by the indexing model from the issue text.