fixed parenthesis missing + wrong name for a self reference

shape.lua

@@ -47,10 +47,10 @@ end
 
 -- Sets this face's normal to be the average of its vertices, normalized
 function face:average_normal()
-  local fvs = f.vertices
-  f.normal = vector.zero(3)
-  for i = 1, #fvs do f.normal = f.normal + fvs[i] end
-  f.normal = (1 / #fvs) * f.normal
+  local fvs = self.vertices
+  self.normal = vector.zero(3)
+  for i = 1, #fvs do self.normal = self.normal + fvs[i] end
+  self.normal = (1 / #fvs) * self.normal
   return self
 end
 
@@ -227,7 +227,7 @@ function shape.hexahedron(r)
 
   local fs = {face.new({-v1,  v2, -v3,  v4}), face.new({v1, -v2,  v3, -v4}),
               face.new({-v1,  v3, -v4,  v2}), face.new({v1, -v3,  v4, -v2}),
-              face.new({-v1,  v4, -v2,  v3}), face.new({v1, -v4,  v2, -v3}}
+              face.new({-v1,  v4, -v2,  v3}), face.new({v1, -v4,  v2, -v3})}
 
   for i = 1, #fs do fs[i]:average_normal() end
   return fs