Update spec/locale.html

Co-authored-by: Richard Gibson <richard.gibson@gmail.com>

Author: FrankYFTang

spec/locale.html

@@ -841,7 +841,7 @@ <h1>
       </h1>
       <dl class="header">
         <dt>description</dt>
-        <dd>It converts a <a href="https://unicode.org/reports/tr35/#UnicodeFirstDayIdentifier">Unicode First Day Identifier</a> defined in <a href="https://unicode.org/reports/tr35/#Key_And_Type_Definitions_">Unicode Technical Standard #35 Part 1 Core Section 3.6.1 Key and Type Definitions</a> to a numeric ISO 8601 <em>calendar day of week</em>, and returns any other string unmodified.</dd>
+        <dd>It converts a <a href="https://unicode.org/reports/tr35/#UnicodeFirstDayIdentifier">Unicode First Day Identifier</a> defined in <a href="https://unicode.org/reports/tr35/#Key_And_Type_Definitions_">Unicode Technical Standard #35 Part 1 Core Section 3.6.1 Key and Type Definitions</a> to a numeric ISO 8601 <em>calendar day of week</em>, and returns *undefined* for any other string.</dd>
       </dl>
       <emu-alg>
         1. For each row of <emu-xref href="#table-locale-first-day-option-value"></emu-xref>, except the header row, in table order, do