docs: make `Table of contents` as bold
diff --git a/docs/select-expression.md b/docs/select-expression.md
index 17f3dbb..cf39b7b 100644
--- a/docs/select-expression.md
+++ b/docs/select-expression.md
@@ -17,7 +17,7 @@
 --> 
 
 
-## Table of contents
+**Table of contents**
 
 <!--- TOC -->