Editorial: Tweak two SV rules

Where the SV of a production only depends on
the MV of the production's LHS,
change it to use the 'parent' production,
so that SV doesn't reach deeper than it needs to.

(This commit would have been more at home in PR tc39#2018.)

spec.html

@@ -14226,10 +14226,10 @@ <h1>Static Semantics: SV</h1>
             The SV of <emu-grammar>NonOctalDecimalEscapeSequence :: `9`</emu-grammar> is the String value consisting of the code unit 0x0039 (DIGIT NINE).
           </li>
           <li>
-            The SV of <emu-grammar>HexEscapeSequence :: `x` HexDigit HexDigit</emu-grammar> is the String value consisting of the code unit whose value is the MV of |HexEscapeSequence|.
+            The SV of <emu-grammar>EscapeSequence :: HexEscapeSequence</emu-grammar> is the String value consisting of the code unit whose value is the MV of |HexEscapeSequence|.
           </li>
           <li>
-            The SV of <emu-grammar>Hex4Digits :: HexDigit HexDigit HexDigit HexDigit</emu-grammar> is the String value consisting of the code unit whose value is the MV of |Hex4Digits|.
+            The SV of <emu-grammar>UnicodeEscapeSequence :: `u` Hex4Digits</emu-grammar> is the String value consisting of the code unit whose value is the MV of |Hex4Digits|.
           </li>
           <li>
             The SV of <emu-grammar>UnicodeEscapeSequence :: `u{` CodePoint `}`</emu-grammar> is the result of performing UTF16EncodeCodePoint on the MV of |CodePoint|.