doc/extension.rdoc: Fix small copy+paste mistake

[Fix GH-1884]

From: Lars Kanis <lars@greiz-reinsdorf.de>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63622 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

doc/extension.rdoc

@@ -746,7 +746,7 @@ be assigned to sval, which should be a pointer of the type specified.
 ==== Ruby object to C struct
 
 To retrieve the C pointer from the Data object, use the macro
-Data_Get_Struct().
+TypedData_Get_Struct().
 
   TypedData_Get_Struct(obj, type, &data_type, sval)