Corrections for 2.6 to 2.7 upgrade docs: wildcard-router and layout

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.7/html/migrating_3scale/upgrading-3scale#procedure

1. At step 6 of the paragraph 2.3.1 linked above the sentence `and checking that the ‘tags’ column shows the tag 2.7.` is included in the dark box that should only enclose the command.
2. Step 9 of the same paragraph: `apicast-wildcard-router` is included in the `THREESCALE_DC_NAMES` variable although this component was removed in 2.6. This is not blocking but results in an (harmless) error during this step.