docs, tutorial, typo

[Jacek Wotka <cn team-fatal com>]

hi, while reading chapter 4 section 2 of your tutorial 
(http://gtkmm.sourceforge.net/docs/gtkmm-2.4/docs/tutorial/html/ch04s02.html), 
i stumbled on this typo in the second paragraph "ToggelButton", so i 
though i refer to the first page, where it says "We would very much 
appreciate any reports of inaccuracies or other errors in this 
document.". apart from this, very good work so far.