.gitignore: fix !doc/build pattern
.gitignore: fix !doc/build pattern

There is no doc/guides/build directory. Fix a line referencing this to
refer to the actual directory containing build and configuration
system documentation, doc/build. (This is distinct from the
documentation build directory, which is canonically doc/_build. I
know...).

Signed-off-by: Martí Bolívar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

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