Add a built-in template call test and make example in docs simpler

docs/language.md

@@ -988,13 +988,13 @@ Similarly, template function calls can be used in functions to wrap a sequence
 of expressions. For example:
 
 ```flitter
-func onoise(seed, n, k, x, y, z)
+func onoise(seed, n, k, x, y)
     let weights=k**i for i in ..n
         total=sum(weights)
     @sum
         for i in ..n
             let scale=2**i
-            noise(seed, x*scale, y*scale, z*scale) * weights[i] / total
+            noise(seed, x*scale, y*scale) * weights[i] / total
 ```
 
 This function is (almost) equivalent to the built-in function `octnoise()` and

tests/test_language.py

@@ -316,6 +316,20 @@ def test_template_calls(self):
   !baz #test
             """)
 
+    def test_template_builtin_calls(self):
+        self.assertCodeOutput(
+            """
+func foo(n)
+    @sum
+        for i in ..n
+            2**i
+
+!foo foo=foo(10)
+            """,
+            """
+!foo foo=1023
+            """)
+
     def test_nested_functions(self):
         self.assertCodeOutput(
             """