docs: Use consistent preset for example

Snippets in the later sections for running examples and tests use
`iotsdk-example-rtx.elf` built by the preset `cmsis-rtx-an552`, so the
section for building a preset should use the same preset to avoid
confusion.

Remove `--target iotsdk-example-freertos` because this FreeRTOS example
is not built by the preset for RTX, and `--target` is not compulsory and
requires knowing the CMake target beforehand which is unlikely to be the
case for someone who is new to CMake.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>