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)