1.5.3 release notes nit

Says Documentation/RelNotes-1.5.3.txt:

 - core.legacyheaders is no more, although we still can read
   objects created in a new loose object format.

Someone who doesn't know the background might mistake the 'new' as a
typo, reading it as 'old' - since usually legacy stuff is removed, not
reinstantiated.

Better perhaps:

  - core.legacyheaders is no more, the legacy format of loose objects
    is again "the" format; the previously "new" format is not used
    anymore, although we still can read objects created in that format.

-- Hannes