misplaced backticks in the compile_dll() help page

[mcol]

Note the last part of the sentence:

     If this is used to compile code that uses Rcpp, you will need to
     add the following line to your ‘Makevars’ file so that it knows
     where to find the Rcpp headers: PKG_CPPFLAGS=$(R_HOME)/bin/Rscript
     -e 'Rcpp:::CxxFlags()'``

Instead, the following should appear:

PKG_CPPFLAGS=`$(R_HOME)/bin/Rscript -e 'Rcpp:::CxxFlags()'`