update "principles" to "patterns" in other docs/references

[srabun]

editorial process doc
A/V charter
any A/V or Discovery related materials

[zimeon]

There are links in:

source/_posts/2017-02-28-newsletter.md:[iiif-design]: /api/annex/notes/design_patterns/
source/api/annex/index.html:  <li><a href="/api/annex/notes/design_patterns/">IIIF Design Patterns</a></li>
source/api/annex/notes/index.html:  <li><a href="design_patterns/">IIIF Design Patterns</a></li>
source/community/groups/av/charter.md:[design-principles]: http://iiif.io/api/annex/notes/design_patterns/
source/community/groups/av/index.md:[design-principles]: http://iiif.io/api/annex/notes/design_patterns/

and I see the text "Design Principles" in:

simeon@Cider iiif.io> grep -ir 'Design Principles' source
source/_posts/2017-02-28-newsletter.md:### IIIF Design Principles
source/_posts/2017-02-28-newsletter.md:The IIIF Editors have published a list of [IIIF design principles][iiif-design] that serve to guide the development of IIIF technical specifications.
source/community/groups/av/charter.md:The IIIF A/V Technical Specification Group will create technical specifications that enable interoperability of access to audio and video content following the same [design principles][design-principles] as existing IIIF specifications, allowing the integration of that content with the Presentation API. The group will assist with and steer the implementation of community infrastructure, such as reference implementations and validators, as well as transformation tools to generate the required data from existing systems and APIs.
source/community/groups/av/index.md:While IIIF has focused primarily on Image based content, there has always been interest around the community in extending the paradigm to include Video and Audio resources, following the same [design principles][design-principles] as existing IIIF specifications.