fixed index entry

--- a/doc-src/TutorialI/fp.tex	Mon Aug 08 07:35:42 2011 +0200
+++ b/doc-src/TutorialI/fp.tex	Mon Aug 08 08:25:28 2011 +0200
@@ -249,7 +249,7 @@
 
 \index{type synonyms}%
 Type synonyms are similar to those found in ML\@. They are created by a 
-\commdx{type\_synonym} command:
+\commdx{type\protect\_synonym} command:
 
 \medskip
 \input{Misc/document/types.tex}