porting guide: fix path
porting guide: fix path

Markdown format is no longer used (superseded by rst format, see [1])

Link: [1] https://github.com/ARM-software/arm-trusted-firmware/commit/6f6257476754
Signed-off-by: Pierre-Jean Texier <a4ee2cabad121dc5e19995f1ded171cdfaa10b66@gmail.com>
Reviewed-by: Joakim Bech <741f90e2a7f4d9afbad8bd50be20560f4cf38962@linaro.org>
[jf: shorten URL and move it to a Link: tag]
Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os