[MC] Fix typo: .symtab_shndxr -> .symtab_shndx

This special section is named .symtab_shndx, according to gABI Chapter 4
Sections, and the name is used by some other tools. Though the section
type SHT_SYMTAB_SHNDX is what really matters, let's fix the typo
introduced in rL204769 :)

llvm-svn: 358247