docs: Upgrade make properclean to make distclean

[ThestralWarrior]

Using make distclean avoids issues created from residual build artifacts.
After building for x86_64 architecture, the user follows the guide and uses make properclean which only gets rid of build and not the config which leads to the following error when building for arm64 -

> /home/ashirvad/workdir/helloworld/workdir/unikraft/plat/common/include/x86/irq.h: In function ‘irqs_disabled’:
> /home/ashirvad/workdir/helloworld/workdir/unikraft/plat/common/include/x86/irq.h:61:28: error: ‘X86_EFLAGS_IF’ undeclared (first use in this function)
>    61 |                 x = (__f & X86_EFLAGS_IF) ? 1 : 0; \
>       |                            ^~~~~~~~~~~~~
> /home/ashirvad/workdir/helloworld/workdir/unikraft/plat/common/include/x86/irq.h:82:9: note: in expansion of macro ‘__save_flags’
>    82 |         __save_flags(flag);
>       |         ^~~~~~~~~~~~
> /home/ashirvad/workdir/helloworld/workdir/unikraft/plat/common/include/x86/irq.h:61:28: note: each undeclared identifier is reported only once for each function it appears in
>    61 |                 x = (__f & X86_EFLAGS_IF) ? 1 : 0; \
>       |                            ^~~~~~~~~~~~~
> /home/ashirvad/workdir/helloworld/workdir/unikraft/plat/common/include/x86/irq.h:82:9: note: in expansion of macro ‘__save_flags’
>    82 |         __save_flags(flag);
>       |         ^~~~~~~~~~~~