add an example of semicolon supression

examples/example.jl

@@ -71,6 +71,9 @@ end
 
 foo()
 
+# Just like in the REPL, outputs ending with a semicolon hide the output:
+1 + 1;
+
 # Both Documenter's `@example` block and notebooks can display images. Here is an example
 # where we generate a simple plot using the
 # [Plots.jl](https://github.com/JuliaPlots/Plots.jl) package