fixups: ubdef instead of ubxref in basic.tex

Author: joshuaberne

source/basic.tex

@@ -4423,7 +4423,7 @@
 signature.
 
 \pnum
-If a deallocation function terminates by throwing an exception, the behavior is undefined.\ubxref{basic.stc.alloc.dealloc.throw}
+If a deallocation function terminates by throwing an exception, the behavior is undefined.\ubdef{basic.stc.alloc.dealloc.throw}
 The value of the first argument supplied to a deallocation function may
 be a null pointer value; if so, and if the deallocation function is one
 supplied in the standard library, the call has no effect.