Documentation: getting_started/gpio.md: fix markup

Change-Id: I2c61770d60a4f290fd8d516850f16bc3808ad48d
Signed-off-by: Ivan Labáth <iger@labo.rs>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39082
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/getting_started/gpio.md

@@ -25,7 +25,7 @@ how to appropriately set these registers. In addition, some mainboards are
 based on a baseboard/variant model, where several variant mainboards may share a
 lot of their circuitry and ICs and the commonality between the boards is
 collected into a virtual ``baseboard.`` In that case, the GPIOs which are shared
-between multiple boards are placed in the baseboard's ``gpio.c` file, while the
+between multiple boards are placed in the baseboard's ``gpio.c`` file, while the
 ones that are board-specific go into each variant's ``gpio.c`` file.
 
 ## Intel SoCs