Update intro.tex

Author: AbleBacon

source/intro.tex

@@ -173,7 +173,7 @@
 
 \indexdefn{program!ill-formed}%
 \definition{ill-formed program}{defns.ill.formed}
-program that is not well formed
+program that is not well-formed~(\ref{defns.well.formed})
 
 \indexdefn{behavior!implementation-defined}%
 \definition{implementation-defined behavior}{defns.impl.defined}