inconsistent namespace references

* sometimes `http://www.opendatakit.org/...`
* sometimes `http://opendatakit.org/...`

```
$ git grep opendatakit.org
CHANGELOG.md:* New preferred http://opendatakit.org/xforms namespace for new functionality.
_sections/10-structure.md: xmlns:odk="http://www.opendatakit.org/xforms"
_sections/100-encryption.md:
_sections/100-encryption.md:The following elements are supported. Unless otherwise specified all attributes and elements should be in the `"http://www.opendatakit.org/xforms/encrypted"` namespace.
_sections/120-entities.md:
_sections/120-entities.md:
_sections/120-entities.md:This specification uses the `http://www.opendatakit.org/xforms/entities` namespace for attributes added to nodes defined by the ODK XForms spec. In this document, the corresponding prefix used is `entities`.
_sections/120-entities.md:Consumers of the ODK XForms specification may opt into this entities specification but don't have to. For this reason, the entities layer is versioned separately from ODK XForms using the `entities-version` attribute in the `http://www.opendatakit.org/xforms/entities` namespace.
_sections/15-namespaces.md:Elements and attributes that are specific to ODK XForms and not defined by the XForms 1.0 specification should be separately namespaced. For historical reasons, the `"http://openrosa.org/javarosa"` namespace (with the `jr` prefix in this document), and the `"http://openrosa.org/xforms"` namespace (with the `orx` prefix in this document) have been used. For any new additions not defined in another specification, the `"http://www.opendatakit.org/xforms"` namespace is now preferred. It is assigned the `odk` prefix in this documentation. If a new feature is copied from another XForms implementation the originator's namespace will be used.
_sections/20-instance.md:| `odk:prefix` | on the childnode of the primary instance in the _http://opendatakit.org/xforms_ namespace: optional string prefix which is included at the beginning of the [compact representation](#compact-record-representation-(for-sms))
_sections/20-instance.md:| `odk:delimiter` | on the childnode of the primary instance in the _http://opendatakit.org/xforms_ namespace: optional string delimiter which is used to separate prefix, tags and values in the [compact representation](#compact-record-representation-(for-sms))
_sections/20-instance.md:| `odk:tag` | on a question node (grandchild of the primary instance) in the _http://opendatakit.org/xforms_ namespace: optional string tag which is used to identify nodes that should be part of the [compact representation](#compact-record-representation-(for-sms))
```

Start with the files listed by `git grep opendatakit.org`, especially `_sections/10-structure.md`, `_sections/100-encryption.md`, `_sections/120-entities.md`, `_sections/15-namespaces.md`, and `_sections/20-instance.md`. Compare each namespace reference with the preferred form described in the issue, update inconsistent documentation examples and prose, then rerun the grep to confirm the references are consistent.

Written by the indexing model from the issue text.