change the subsection level for empty matrices and empty arrays

Author: avehtari

src/reference-manual/expressions.qmd

@@ -301,7 +301,7 @@ The empty expression `[ ]` is ambiguous and therefore is not
 allowed and similarly expressions such as `[ [ ] ]` or
 `[ [ ], [ ] ]` are not allowed.
 
-#### Empty vectors and matrices {-}
+### Empty vectors and matrices {-}
 
 If needed, it is possible to create an empty vector with
 ```stan
@@ -347,7 +347,7 @@ array[2, 3] int b = { { 1, 2, 3 },
                 { 4, 5, 6 } };
 ```
 
-#### Empty arrays {-}
+### Empty arrays {-}
 
 The empty array expression (`{ }`) is not allowed. See more about restrictions on array expressions in subsection [Restrictions on values](#array-expressions).