Fix language describing protected methods

Fixes [Bug #18887]

doc/syntax/modules_and_classes.rdoc

@@ -155,7 +155,7 @@ Ruby has three types of visibility.  The default is +public+.  A public method
 may be called from any other object.
 
 The second visibility is +protected+.  When calling a protected method the
-receiver must inherit the Class or Module which defines the method.  Otherwise a
+sender must inherit the Class or Module which defines the method.  Otherwise a
 NoMethodError will be raised.
 
 Protected visibility is most frequently used to define <code>==</code> and