diff --git a/doc/reference.rst b/doc/reference.rst
index 865ee8fbe110bdf2670eaae7b478df6e8e87a431..a35cf510ef147090d68dd78a8ea103726da630f1 100644
--- a/doc/reference.rst
+++ b/doc/reference.rst
@@ -203,7 +203,7 @@ C Block Instructions
 Substitution Rules
 ^^^^^^^^^^^^^^^^^^
 
-Syntax of an substitution rule::
+Syntax of a substitution rule::
 
     rule_name(arg1, arg2) := EXPRESSION