[lldb] Fix typos and update "GDB To LLDB Command Map" to be a bit more clear

I've gone through the GDB To LLDB Command Map and tried to improve it:

 - Fix obvious typos (e.g. <cope> → <code>)
 - Wrap code and program names in <code> tags
 - Reword a couple parts where (IMHO) the phrasing could be a bit better

Differential revision: https://reviews.llvm.org/D28758