doc: add missing self qualifier to (Person) -getAgeInUnits:

doc/sample/Person.im

@@ -87,9 +87,9 @@ class Person
 
     - getAgeInUnit:units
         ^match units in
-            #years  => age,
+            #years  => self.age,
-            #months => age / 12,
+            #months => self.age / 12,
-            #days   => age / 365,
+            #days   => self.age / 365,
             _       => 0,
         end
     end