Fix documentation of @benchmark (#203)

The docstring of `@benchmark` is a raw string literal,
hence `@doc` is required.

Author: FedericoStra

src/execution.jl

@@ -321,7 +321,7 @@ function quasiquote!(ex::Expr, vars::Vector{Expr})
     return ex
 end
 
-raw"""
+@doc raw"""
     @benchmark <expr to benchmark> [setup=<setup expr>]
 
 Run benchmark on a given expression.