Merge pull request #150 from ascherer/fix-typo-in-book-sample

Run 'aspell' on book sample.

sample-book.tex

@@ -920,7 +920,7 @@ \section{Numbered Section Headings}
 \index{headings!numbered}
 
 While Tufte dispenses with numbered headings in his books, if you require them,
-they can be anabled by changing the value of the \doccounter{secnumdepth}
+they can be enabled by changing the value of the \doccounter{secnumdepth}
 counter.  From the table below, select the heading level at which numbering
 should stop and set the \doccounter{secnumdepth} counter to that value.  For
 example, if you want parts and chapters numbered, but don't want numbering for