[basic.link] Fix cross-reference to translation unit

Tranalation units are defined in phase 7 of translation,
[lex.phases] not [lex.separate].

Author: AlisdairM

source/basic.tex

@@ -2886,7 +2886,7 @@
 \pnum
 \indextext{program}%
 \indextext{linking}%
-A \defn{program} consists of one or more translation units\iref{lex.separate}
+A \defn{program} consists of one or more translation units\iref{lex.phases}
 linked together. A translation unit consists
 of a sequence of declarations.