Typos and small fixes in "Functions" chapter

- change "user created" to "user-created"  
- change a "paste(sep='')" to paste0() (paste0 is indeed used in the next paragraph)
- change side-effects to "side effects" (it is used non-hyphenated the rest of the times in the chapter)
Merge pull request #637 from daattali/patch-3

Typos and small fixes in "Functions" chapter