Improve title of 4274

Author: jwakely

xml/issue4274.xml

@@ -2,7 +2,7 @@
 <!DOCTYPE issue SYSTEM "lwg-issue.dtd">
 
 <issue num="4274" status="New">
-<title>The `chrono::hh_mm_ss` constructor is poorly defined for negative durations</title>
+<title>The `chrono::hh_mm_ss` constructor is ill-formed for unsigned durations</title>
 <section>
 <sref ref="[time.hms.members]"/>
 </section>