ui/cocoa: Update path to docs in build tree

QEMU documentation can't be opened if QEMU is run from build tree
because executables are placed in the top of build tree after conversion
to meson.

Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20210108213815.64678-1-r.bolshakov@yadro.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>