Add missing space between \btrue

There was a space missing in some the documentation for
lldb::BreakpointsWriteToFile.

This fixes the following doxygen error when building the lldb-cpp-doc
target:

  llvm-project/lldb/include/lldb/API/SBTarget.h:775 warning: Found
  unknown command `\btrue'

Patch by: Konrad Kleine

Differential revision: https://reviews.llvm.org/D60442

llvm-svn: 357980