[whatwg] Two small typos in UndoManager spec

[Caio Marcelo de Oliveira Filho]

Hi,

Found in 8/June/2012 version.

Section 3.2: the first example box says "In the following example,
execute and undo attributes are modified:" but executeAutomatic and
undo are modified in the actual example.

Section 3.4: "user agnets" should be "user agents", it appear three times.

-- 
Caio Marcelo de Oliveira Filho
openBossa @ INdT - Instituto Nokia de Tecnologia