[mile] some minor coorrections for RFC7970 and its schema

["Takeshi Takahashi" <[e-mail]>]

Hello Roman and all,

Based on the discussion at the Prague meeting, let me propose the following
corrections for RFC7970 and its schema.

[Proposed changes to the IODEFv2 schema registered at the IANA website]
Change #1: unnecessary space in the schemaLocation of
xmldsig-core-schema.xsd was removed
Change #2: unnecessary space in the schemaLocation of iodef-enum-1.0.xsd was
removed
Change #3: unnecessary space in the schemaLocation of iodef-sci-1.0.xsd was
removed
Change #4 The minOccurs of the URL was defined in the ThreatActor class.
(Otherwise, the schema does not allow us to omit the URL element.)
Change #5 the "ext-value" was not supported by the above enum value.
Therefore, the bulkobservable-type-type was fixed to allow the value
"ext-value".

FYI: The attached file is the revised schema file.

[Proposed correction to the RFC7970]
The IODEFv2 schema is embedded in the RFC7970.
Therefore, the above correction needs to be reflected to the RFC7970 as
well.

In addition to that, in the example in Section 7.2, the current text has the
following line.
               <BulkObservable type="fqdn">
However, "fqdn" is not a qualified value. We may use "domain-name" or
"ext-value".

If the schema is going to be revised, we will work on the JSON schema based
on the revised one.

Take