scripts/spelling.txt: add spelling fix for prohibited

Misspelling 'prohibited' is quite common in the real world, although
surprisingly not so much in the Linux Kernel.  In addition to fixing the
typo we may as well add it to the spelling checker.

Also adding the present participle (prohibiting).

Link: http://lkml.kernel.org/r/20190514153341.22540-1-chris.paterson2@renesas.com


Fixes: 5bf2fbbe ("clk: renesas: cpg-mssr: Add r8a77470 support")
Signed-off-by: Chris Paterson <chris.paterson2@renesas.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>