[lex.phases] Use consistent (maths) style for reference points in example

Author: tkoeppe

source/lex.tex

@@ -261,24 +261,24 @@
 
   class Inner {
     void fn() {
-      /* P1 */ Incomplete i;    // OK
+      /* @$p_1$@ */ Incomplete i;    // OK
     }
-  }; /* P2 */
+  }; /* @$p_2$@ */
 
   consteval {
     define_aggregate(^^Incomplete, {});
   }
-}; /* P3 */
+}; /* @$p_3$@ */
 \end{codeblock}
-Constructs at P1 are analyzed in a context
+Constructs at $p_1$ are analyzed in a context
 where the side effect of the call to \tcode{define_aggregate} is evaluated
 because
 \begin{itemize}
 \item
 $E$ is the expression corresponding to a consteval block, and
 \item
-P1 is in a complete-class context of \tcode{S} and
-the consteval block is reachable from P3.
+$p_1$ is in a complete-class context of \tcode{S} and
+the consteval block is reachable from $p_3$.
 \end{itemize}
 \end{example}