Fix comment in SnapBuildFindSnapshot

Hi,

We have the following comment in SnapBuildFindSnapshot():

    * c) transition from FULL_SNAPSHOT to CONSISTENT.
    *
    * In FULL_SNAPSHOT state (see d) ), and this xl_running_xacts'

It mentions "(state d) )", which seems like a typo of "(state d)", but
there is no "state d" in the first place. Reading the discussion of
the commit 955a684e040 that introduced this comment, this was a
comment for an old version patch[1]. So I think we can remove this
part.

I've attached the patch.

Regards,

[1] https://www.postgresql.org/message-id/20170505004237.edtahvrwb3uwd5rs%40alap3.anarazel.de

-- 
Masahiko Sawada
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