[docs]fix an item missing (#18717)

doc/astspec.txt

@@ -364,6 +364,7 @@ AST:
     nnkExprColonExpr(nnkIdent("a"), nnkIntLit(1)),
     nnkExprColonExpr(nnkIdent("b"), nnkIntLit(2)),
     nnkExprColonExpr(nnkIdent("c"), nnkIntLit(3)))
+  nnkTupleConstr()
 
 Since the one tuple would be syntactically identical to parentheses
 with an expression in them, the parser expects a trailing comma for