[Tei-council] minor schema-altering corrigible error fixed

I just discovered that the TEI schema had "alternate" as one of the values for classRef/@expand. The prose in the Guidelines, the remarks in the tagdoc, and the Stylesheets all say it should be "alternation". I see this as a corrigible error, and thus have fixed it. It is, however, a schema-changing alteration, and thus should be announced quite up-front in the release notes. BTW, I have checked through all of the Examplars, and none were using either value. See issues #1903 and #1904.