docs(design): close the sma_demo module paren in models/0007 (refs #7)

Verifying the previous commit's surface-syntax fix with `ail parse`
(the whitepaper's code blocks are unfenced/untagged, so no spec
validation ever parsed them) surfaced a fourth, pre-existing drift the
line-by-line signature fix had missed: the §sma_demo `(module …)` was
short one closing paren and never balanced — it could not have parsed
as written, independent of the mode/con-wrapper drift. Added the
missing `)`.

With it, both worked-example modules now parse clean:
- §sma_demo  → `ail parse` ok (4.4 KB JSON)
- §series    → `ail parse` ok

`ail parse` resolves no names, so this verifies surface syntax only,
which is the intended scope. The §series listing still leans on
kernel-tier polymorphism that `ail check` rejects today; its full
compile-verification stays with the pending Series milestone per
spec 0059, exactly as the prior commit stated.

design/models/0007-kernel-extensions.md

@@ -105,7 +105,7 @@ moving average over a stream of float values, window size 3":
           (term-ctor FloatList FCons 8.0
           (term-ctor FloatList FCons 6.0
           (term-ctor FloatList FCons 2.0
-          (term-ctor FloatList FNil)))))))))))
+          (term-ctor FloatList FNil))))))))))))
 ```
 
 Four constructs in this program are not in today's AILang. Each