spec: fix two errors in the dynamic fields example

Since the example declares `a: "foo"`,
`(a): "baz"` is `foo: "baz"` and not `bar: "baz"`.

While here, `a: "foo"` was missing a closing quote.

Fixes #2592.

Closes #2621 as merged as of commit 813f70a.

Signed-off-by: Stephen Herbein <[email protected]>
Change-Id: I4d631833f51581fceb9d08df3b533d931c131463
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1171216
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: SteVwonder

doc/ref/spec.md

@@ -1166,9 +1166,9 @@ A dynamic field may be marked as optional or required.
 
 ```
 Expression                             Result
-a:   "foo                              a:   "foo"
+a:   "foo"                             a:   "foo"
 b:   "bar"                             b:   "bar"
-(a): "baz"                             bar: "baz"
+(a): "baz"                             foo: "baz"
 
 (a+b): "qux"                           foobar: "qux"