Improve fake expansion of %- found in %= docs

The discussion of `%-` in the `%=` section is very valuable but the
macro expansion given there has two deficiencies: 1) it is fake, and
more importantly 2) someone relying on it could get bitten by the
following:

```hoon
=+  a=5
=+  add
=>  %=  +2
      +6  [a 9]
    ==
$
```

This twig produces 9, not 14, because the `a` there resolves to the head
of the sample of `+add`, whereas the supposedly equivalent `(add a
9):[a=5 .]` does produce the expected result of 14.

content/reference/hoon/rune/cen.md

@@ -805,7 +805,7 @@ Here's that again in tall form:
 ```hoon
 =+  foo
 =>  %=  +2
-      +6  baz
+      +6  baz:+3
     ==
   $
 ```