Update xml/issue4207.xml

Co-authored-by: Jonathan Wakely <[email protected]>

Author: tomaszkam

xml/issue4207.xml

@@ -63,7 +63,7 @@ The cross reference to <sref ref="[class.mem]"/> should be changed to
 <sref ref="[class.mem.general]"/>.
 </p>
 <p>
-The treatment of default template argument is subject for CWG
+The treatment of default template arguments is the subject of CWG
 <a href="https://cplusplus.github.io/CWG/issues/1635.html">1635</a>.
 An alternative design, where `source_location` inside default template argument
 refers to point of use. It was noted that this does not reflect current behavior