Documentation/everyday: match undefline with the text

Even though AsciiDoc is more lenient when deciding if an underline
is for the contents on the previous line to find section headers, we
should match the length of them for other formatters to help them.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/giteveryday.txt

 giteveryday(7)
-===============
+==============
 
 NAME
 ----