Editorial: MV: add def for Hex4Digits

PR #984 added a reference to "the MV of |Hex4Digits|"
(in the definition of CharacterValue)
but didn't add a definition for it.

Extract one from the definition for "the SV of |Hex4Digits|".

spec.html

@@ -11261,6 +11261,9 @@ <h1>Static Semantics: MV</h1>
           <li>
             The MV of <emu-grammar>HexDigits :: HexDigits HexDigit</emu-grammar> is (the MV of |HexDigits| &times; 16<sub>ℝ</sub>) plus the MV of |HexDigit|.
           </li>
+          <li>
+            The MV of <emu-grammar>Hex4Digits :: HexDigit HexDigit HexDigit HexDigit</emu-grammar> is (0x1000<sub>ℝ</sub> times the MV of the first |HexDigit|) plus (0x100<sub>ℝ</sub> times the MV of the second |HexDigit|) plus (0x10<sub>ℝ</sub> times the MV of the third |HexDigit|) plus the MV of the fourth |HexDigit|.
+          </li>
         </ul>
       </emu-clause>
 
@@ -11626,7 +11629,7 @@ <h1>Static Semantics: SV</h1>
             The SV of <emu-grammar>UnicodeEscapeSequence :: `u` Hex4Digits</emu-grammar> is the SV of |Hex4Digits|.
           </li>
           <li>
-            The SV of <emu-grammar>Hex4Digits :: HexDigit HexDigit HexDigit HexDigit</emu-grammar> is the code unit whose value is (0x1000<sub>ℝ</sub> times the MV of the first |HexDigit|) plus (0x100<sub>ℝ</sub> times the MV of the second |HexDigit|) plus (0x10<sub>ℝ</sub> times the MV of the third |HexDigit|) plus the MV of the fourth |HexDigit|.
+            The SV of <emu-grammar>Hex4Digits :: HexDigit HexDigit HexDigit HexDigit</emu-grammar> is the code unit whose value is the MV of |Hex4Digits|.
           </li>
           <li>
             The SV of <emu-grammar>UnicodeEscapeSequence :: `u{` CodePoint `}`</emu-grammar> is the UTF16Encoding of the MV of |CodePoint|.