Fix a very minor (but annoying when looking for things!) markup nit.
diff --git a/Doc/ref/ref4.tex b/Doc/ref/ref4.tex
index 754a5b5..2272f3e 100644
--- a/Doc/ref/ref4.tex
+++ b/Doc/ref/ref4.tex
@@ -1,6 +1,7 @@
 \chapter{Execution model \label{execmodel}}
 \index{execution model}
 
+
 \section{Code blocks, execution frames, and namespaces \label{execframes}}
 \index{code block}
 \index{namespace}