[tei-council] rooting out some mentions of SGML in the Guidelines

[Kevin Hawkins]

There are a number of instances in the TEI Guidelines of "SGML or XML". 
  Is it safe to drop the reference to SGML in cases where current 
practice, and not history, is being discussed?  The Guidelines, after 
all, switched to XML syntax with P5, so I can't see any reason to keep 
these.

Also okay to change examples where the value of ref at target contains 
".sgm" to ".xml"?

--K.