[xrefdelta] Fix typo in the entry for [re.def] (#8112)

The contents were moved to [intro.defs], not [intro.refs]. The latter is a typo.

Author: frederick-vs-ja

source/xrefdelta.tex

@@ -300,7 +300,7 @@
 \movedxref{depr.iterator.primitives}{depr.iterator}
 \movedxref{depr.iterator.basic}{depr.iterator}
 
-\movedxref{re.def}{intro.refs}
+\movedxref{re.def}{intro.defs}
 \movedxref{basic.scope.declarative}{basic.scope.scope}
 \movedxref{basic.funscope}{stmt.label}
 \movedxref{basic.scope.hiding}{basic.lookup}