doc: sframe: mention errata 1 of SFrame version 2

With the changes of an added flag SFRAME_F_FDE_FUNC_START_PCREL, s390x
support and new section type SHT_GNU_SFRAME, indicate that this document
specifies the errata 1 of SFrame version 2.  This will help distinguish
the document / specification better from previous releases.

libsframe/doc/
	* sframe-spec.texi: Mention errata 1 of SFrame version 2.