[css-egg] fix typo

Author: frivoal

css-egg/Overview.bs

@@ -267,7 +267,7 @@ are appropriate for very short durations,
 their use to express longer time spans is inadequate.
 Moreover, authors may sometimes wish to use less dry terminology
 and prefer more expressive and vivid language. This specification
-therefore introduces three two <<time>> units:
+therefore introduces two <<time>> units:
 ''ftn'' and its subdivision ''mftn''.
 
 <table class="data" export>