fix Issue 3800 - 'Foreach over Structs and Classes with Ranges' and '…

…Invariant Struct' in D2 Spec

struct.dd

@@ -562,7 +562,7 @@ $(TR $(TD .offsetof) $(TD Offset in bytes of field from beginning of struct))
 )
 
 $(V2
-$(SECTION3 <a name="ConstStruct">Const and Invariant Structs</a>,
+$(SECTION3 <a name="ConstStruct">Const and Immutable Structs</a>,
 
         $(P A struct declaration can have a storage class of
         $(CODE const), $(CODE immutable) or $(CODE shared). It has an equivalent