m.measure manual: mention square feet

fixes https://trac.osgeo.org/grass/ticket/3991

misc/m.measure/m.measure.html

@@ -2,7 +2,7 @@
 
 <em>m.measure</em> provides the user with a way to measure the lengths
 and areas of lines and polygons. Areas can be stated in acres,
-hectares, square miles, square meters and square kilometers.
+hectares, square miles, square feet, square meters and square kilometers.
 
 <h2>EXAMPLES</h2>