Fix malformed mime type

[szh]

Desired Outcome

Fix error

ActionDispatch::Http::MimeNegotiation::InvalidType ("text
plain" is not a valid MIME type)

Implemented Changes

• Changed the "text\plain" (backslash) mime type to "text/plain" (forward slash)

Connected Issue/Story

CyberArk internal issue link: ONYX-17641

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: insert issue ID
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes

[rpothier]

LGTM!