Add exception for admin SVG to vale rule

The following discussion from !49154 (merged) should be addressed:

• @aqualls started a discussion:

  For @marcel.amirault and @eread - we'll want a followup tw-testing issue here. This use of **{admin}** is a valid use of the word admin but it's not accounted for in our Admin.yml test.

  I see two approaches here:

  1. Add this string to Admin.yml so we fix the problem for this one SVG shortcut. (Good if this is the only icon named with a word we test for.)
  2. Learn how to tell Vale to skip everything between **{ and }** - probably a TokenIgnores (https://errata-ai.gitbook.io/vale/getting-started/configuration#tokenignores) but I haven't done this before!