Corrected some grammar and typo's


 up the forest stands' borders(the greenish pixels):   = space before the opening parentheses
 
 You pixel selection     =  "You"  should be "Your"
 
 Temember to review      =  "Temember" should be "Remember"
 
follow the the forest stands' borders   =  double "the"

Set your the polygon layer    = "your the" should be "the" regarding the rest of the sentence

border of the already existing polygon/s.  = superfluous "/" Removed

few points inside polygon 358 making  = should be "357" instead of "358" since 358 is the object we are creating.

the :kbd:`exercise_data\\forestry`  = added "directory"

:menuselection:`Vector --> Table Manger -->  = "Manger should be "Manager"
Merge pull request #354 from DiGro/patch-5

Corrected some grammar and typo's