typo

focusses -> focuses
Extra space removed

removed a white space between a word and a comma
typo

this section discuss -> discusses
typo

four types of environment -> environments
knitr option error = TRUE

omitting error = TRUE does not show "Error: Can't find a" at the first rebinding attempt
typo

"Many bugs subtle and hard to find." -- missing "are"
typo

"if a bug were obvious" -> "was"
typo

Missing "a" article
Merge pull request #316 from tonytonov/master

typos, extra white space, knitr option