doc: env_vars: Fix link to section env_vars_zephyrrc
doc: env_vars: Fix link to section env_vars_zephyrrc

Currently the doc says "above in env_vars_zephyrrc" without a link to
that section.

Signed-off-by: Ruslan Mstoi <348dc8e8f10581f5bc17672b9c406a6d9fcc1430@intel.com>

galak/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr