qapi/migration.json: Fix indentation

qapi/migration.json: Fix indentation

Commits 6a9ad1542065ca0bd54c6 and 9004db48c080632aef23 added some
new text to qapi/migration.json which doesn't fit the stricter
indentation requirements imposed by the rST documentation generator.
Reindent those lines to the new standard.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200810195019.25427-2-peter.maydell@linaro.org>
Signed-off-by: Markus Armbruster <armbru@redhat.com>