Fixes the documentation for HexDigitsUS

HexDigitsUS referred to HexDigitUS, which was not specified.
The undescore was moved to HexDigitUS from HexLetter to bring
it in line with how other integers are specified.

lex.dd

@@ -685,6 +685,10 @@ $(GNAME HexDigitsNoSingleUS):
     $(GLINK HexDigit) $(GLINK HexDigitsUS)
     $(GLINK HexDigitsUS) $(GLINK HexDigit)
 
+$(GNAME HexDigitUS):
+    $(GLINK HexDigit)
+    $(D _)
+
 $(GNAME HexDigit):
     $(GLINK DecimalDigit)
     $(GLINK HexLetter)
@@ -702,7 +706,6 @@ $(GNAME HexLetter):
     $(B D)
     $(B E)
     $(B F)
-    $(D _)
 )
 
 	$(P Integers can be specified in decimal, binary, octal, or hexadecimal.)