[AsmPrinter] Rename a comment of .debug_gnu_pubnames entry

Summary:
The comment refers to the field as "Kind:". However, in gdb,

https://sourceware.org/gdb//onlinedocs/gdb/Index-Section-Format.html names it "attributes",
gdb/dwarf2read.c:dw2_symtab_iter_next refers to the whole value as "cu_index_and_attrs"

Change it to `Attributes:` for consistency.

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: aprantl, JDevlieghere, arphaman, llvm-commits

Differential Revision: https://reviews.llvm.org/D54480

llvm-svn: 346790