Three small typos

line 7  : new raster layers usig mathematical expressions  ----> 'usig' should be 'using'

line 7  : A similar algorithm is available for vector layer  ---> 'layer' should be 'layers'(plural)

line 46 : The parameters windows ----> 'windows' should be 'window'(singular)
Merge pull request #424 from DiGro/patch-1

Three small typos