[Defect]: Fix typos in BOM 1.6/1.7 schemas and XSD

[Mehrn0ush]

Describe the defect

There are two minor typos in released CycloneDX BOM 1.6/1.7 artifacts:

1. In JSON Schema and XSD content-type descriptions, "text/plain" for plan text documents should read "plain text".

2. In the refType $comment, staring with 'urn:cdx:' should read starting with 'urn:cdx:'.

Additional context

Evidence (current master):

• schema/bom-1.6.schema.json: contains plan text and staring with
• schema/bom-1.7.schema.json: contains plan text and staring with
• schema/bom-1.6.xsd: contains plan text
• schema/bom-1.7.xsd: contains plan text
  Proto files already use "plain text", so this is a copy-edit consistency fix only.