Updates format to be a single string

Why are these changes being introduced:

* This was a bug in the graphql spec. TIMDEX has always treated format
  as a single string

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/GDT-151

How does this address that need:

* Updates GraphQL spec to correct the expectation to be a single string

Document any side effects to this change:

* None of the TIMDEX sources mapped to anything other than a single
  string for `format`. Therefore, anyone requesting `format` via GraphQL
  would be getting an error in production at this time. This resolves
  that problem, so a deprecation does not feel appropriate. This was a
  bug that caused the inability to use a field, not a change in how a
  previously usable field is being mapped.
* Even though the previous comment remains true, we'll still need to
  refresh the spec in our deployed timdex ui instances as the spec has
  indeed changed.

Author: JPrevost

app/graphql/types/record_type.rb

@@ -163,7 +163,7 @@ class RecordType < Types::BaseObject
     field :contents, [String], null: true, description: 'Table of contents for item'
     field :summary, [String], null: true,
                               description: 'Summary of contents of item (also where abstract goes if applicable)'
-    field :format, [String], null: true, description: 'Format of item e.g. "Print Volume", "DVD", etc.'
+    field :format, String, null: true, description: 'Format of item e.g. "Print Volume", "DVD", etc.'
     field :literary_form, String, null: true, description: 'Identifies the item as fiction or nonfiction'
     field :related_place, [String], null: true, deprecation_reason: 'Use `locations`'
     field :in_bibliography, [String], null: true, deprecation_reason: 'Use `related_items`'