[intro.races] Markup definition of "happens after" (#8648)

Author: lprv

source/basic.tex

@@ -6765,7 +6765,7 @@
 
 \pnum
 An evaluation $A$ \defn{happens before} an evaluation $B$
-(or, equivalently, $B$ happens after $A$)
+(or, equivalently, $B$ \defn{happens after} $A$)
 if either
 \begin{itemize}
 \item $A$ is sequenced before $B$, or