Update Expressions.rmd

The main title for the three chapters is "Metaprogramming", but the title is "Metaprogramming" for this chapter when the link says "Expressions"?

Since I assume #metaprogamming is a label with possible cross references, I don't update this.

Other chapters where the link and title differs are:
Exceptions and debugging (link name)
Profiling (link name)
Rcpp (link name)
Merge pull request #583 from rrunner/patch-12

Update Expressions.rmd