fix the sed command in the makefile

The "-i" option to "sed" to specify in-place editing has a
different syntax under Mac OS X, so should not be used.

Here is the error message:

    make
    echo '% hott-arxiv.tex AUTOGENERATED FROM hott-online.tex AND main.tex' >hott-arxiv.tex
    echo '\pdfoutput=1' >>hott-arxiv.tex
    cat hott-online.tex >>hott-arxiv.tex
    sed -i 's/\\input{main}//' hott-arxiv.tex
    sed: 1: "hott-arxiv.tex": extra characters at the end of h command
    make: *** [hott-arxiv.tex] Error 1

Here is some other info:

    $ uname -a
    Darwin w150158.wireless.ias.edu 13.0.2 Darwin Kernel Version 13.0.2: Sun Sep 29 19:38:57 PDT 2013; root:xnu-2422.75.4~1/RELEASE_X86_64 x86_64

    $ sed -help
    sed: illegal option -- h
    usage: sed script [-Ealn] [-i extension] [file ...]
	   sed [-Ealn] [-i extension] [-e script] ... [-f script_file] ... [file ...]

    $ type sed
    sed is hashed (/usr/bin/sed)
typos in Lemma 6.12.7, close #573
unmarked errata for 6dce346
Merge branch 'master' of https://github.com/DanGrayson/book
explain more about justifications for equalities, close #578
typo in proof of lemma 2.7.2
unmarked errata for ba035f6
oops
clarify and fix typos in lemma 6.10.8
unmarked errata for 13302de
Merge pull request #575 from mikeshulman/lemma6.12.7

typos in Lemma 6.12.7, close #573
Merge pull request #579 from mikeshulman/justifications

explain more about justifications for equalities, close #578
Merge pull request #580 from mikeshulman/refl

typo in proof of lemma 2.7.2
Mark Errata (auto)
some \nameless's should be \blank's.
slight clarifications
change duplicate f's to k's in section 6.12, close #582
unmarked errata for e147418
oops
Merge pull request #585 from mikeshulman/nameless

some \nameless's should be \blank's.