[lex.phases] Update cross-reference into [expr.const] following 98a668e.

The editorial change was made after the wording of P4143R0 was written.

Addresses editorial review committee feedback.

Author: tkoeppe

source/lex.tex

@@ -230,7 +230,7 @@
 The program is ill-formed if any instantiation fails.
 
 During the analysis and translation of tokens,
-each manifestly constant-evaluated expression is evaluated\iref{expr.const}.
+each manifestly constant-evaluated expression is evaluated\iref{expr.const.defns}.
 The values of these expressions affect
 the types named and reflection values used in the program and
 thus the syntactic analysis.