[intro] Fix index entry that uses !see instead of |see.

Author: Eelis

source/intro.tex

@@ -2,7 +2,7 @@
 \rSec0[intro]{General}
 
 \indextext{diagnostic message|see{message, diagnostic}}%
-\indexdefn{conditionally-supported behavior!see{behavior, conditionally-supported}}%
+\indexdefn{conditionally-supported behavior|see{behavior, conditionally-supported}}%
 \indextext{dynamic type|see{type, dynamic}}%
 \indextext{static type|see{type, static}}%
 \indextext{ill-formed program|see{program, ill-formed}}%