Three small typos

line 10 :  it can be defined manually but entering the four values --->  'but' should be 'by'
 
 line 42 :  to execute our rasterization algorith. ----> 'algorith' should be 'algorithm'
  
line 52 :  might not be avaialble ------>'avaialble' should be 'available'
Merge pull request #425 from DiGro/patch-2

Three small typos