fix several typos in code and sentences

[youngroklee-ml]

In "Summary" section in Chapter 15, I believe "timed evaluation" was a typo and needs to be changed to "timed invalidation" to be consistent with a section name in Chapter 15.

[youngroklee-ml]

Also, in Chapter 9 "Downloading Reports" section, in the code chunk that illustrates use of callr package, input$slider in a server function needs to be replaced with input$n to make it work together with ui defined in a previous code chunk.