Typo corrections

Two changes:

*Set code chunk lines 37-43 to be {r, eval = FALSE}, as it was not giving desired output (was showing <environment: 0x1034b2268> after calling both f and environment(f), when it should've been suppressed and showing <environment: R_GlobalEnv>, respectively). Manually inserted comments with desired output.
*Removed two bullet points (just the bullets not the content) from lines 517-519 as a minor formatting fix.

I assign the copyright of this contribution to Hadley Wickham.
Merge pull request #198 from liangcj/patch-2

Typo corrections