Fix ambiguous uses in doctests.

Not sure if these are all the best ways to handle these, but I feel like it's important to have passing doctests.

There's one that still fails on 7.6 (?):

````
### Failure in src/Data/Aeson/Lens.hs:255: expression `_Object._Wrapped # [("key" :: Text, _String # "value")]'
expected: "{\"key\":\"value\"}"
 but got: 
          <interactive>:1004:1:
              No instance for (AsValue t0) arising from a use of `_Object'
              The type variable `t0' is ambiguous
              Possible fix: add a type signature that fixes these type variable(s)
              Note: there are several potential instances:
                instance AsValue String -- Defined at src/Data/Aeson/Lens.hs:276:10
                instance AsValue ByteString
                  -- Defined at src/Data/Aeson/Lens.hs:272:10
                instance AsValue Value -- Defined at src/Data/Aeson/Lens.hs:268:10
              In the first argument of `(.)', namely `_Object'
              In the first argument of `(#)', namely `_Object . _Wrapped'
              In the expression:
                _Object . _Wrapped # [("key" :: Text, _String # "value")]### Failure in src/Data/Aeson/Lens.hs:255: expression `_Object._Wrapped # [("key" :: Text, _String # "value")]'
expected: "{\"key\":\"value\"}"
````