arch: riscv32: Fix trivial comment
arch: riscv32: Fix trivial comment

The Kconfig sybmol referenced in the comment is called
CONFIG_RISCV_SOC_CONTEXT_SAVE not CONFIG_RISCV_SOC_CONTEXT.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

galak/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr