reference: fix anchor link

Author: tshepang

src/doc/reference.md

@@ -3897,7 +3897,7 @@ references to boxes are dropped.
 ### Variables
 
 A _variable_ is a component of a stack frame, either a named function parameter,
-an anonymous [temporary](#lvalues,-rvalues-and-temporaries), or a named local
+an anonymous [temporary](#lvalues-rvalues-and-temporaries), or a named local
 variable.
 
 A _local variable_ (or *stack-local* allocation) holds a value directly,