[libcxx] [test] Fix comment typos, strip trailing whitespace.

No functional change, no code review.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292434 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/function.objects/func.wrap/func.wrap.func/func.wrap.func.con/F_nullptr.pass.cpp

@@ -31,7 +31,7 @@
 //    types can be null. The other categories are not tested here.
 //  3) '__not_null(Callable)' is well formed when the call signature includes
 //      varargs.
-//  4) '__not_null(Callable)' works for Callable types with all aritys less
+//  4) '__not_null(Callable)' works for Callable types with all arities less
 //     than or equal to 3 in C++03.
 //  5) '__not_null(Callable)' works when 'Callable' is a member function
 //     pointer to a cv or ref qualified function type.