Fix doxygen warning about function argument

This fixes the following doxygen warning when building the lldb-cpp-doc
target.

This commit fixes:
  SBStructuredData.h:94 warning: Found unknown command `\dst'
  SBStructuredData.h:97 warning: Found unknown command `\dst'
  SBStructuredData.h:98 warning: Found unknown command `\dst'
  SBStructuredData.h:100 warning: Found unknown command `\dst'
  SBStructuredData.h:104 warning: Found unknown command `\dst'

Patch by: Konrad Kleine

Differential revision: https://reviews.llvm.org/D60443

llvm-svn: 357983