[PDB] Fix a bug in the explain subcommand.

We were trying to dig into the super block fields and print a
description of the field at the specified offset, but we were
printing the wrong field due to an off-by-one-field-error.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328804 91177308-0d34-0410-b5e6-96231b3b80d8