remove `rbenv install` example; clarify manual compile
remove `rbenv install` example; clarify manual compile

It can be confusing if people mistake the example as instructions to
install ruby-build with.

reyesyang/rbenv,josegonzalez/rbenv,hlecuanda/pyenv,cehoffman/rbenv,stivenrgalvis/rbenv,avdhoweb/rbenv,howtolearntocode/rbenv,hlecuanda/pyenv,andrew-christianson/rbenv,dnsd/pyenv,zmwangx/pyenv,cehoffman/pyenv,howtolearntocode/rbenv,princeofdarkness76/rbenv,BanzaiMan/pyenv,pyenv/pyenv,talentdeficit/erlenv,cehoffman/nodenv,andrew-christianson/rbenv,golib/goenv,docwhat/rbenv,VShangxiao/pyenv,princeofdarkness76/rbenv,yeonsh/pyenv,habinmusic/pyenv,docwhat/rbenv,BanzaiMan/pyenv,intfrr/rbenv,cyril/rbenv,silasb/rbenv,takluyver/pyenv,jeremy/rbenv,Ferocia/rbenv,LawnGnome/rbenv,jrolfs/nodenv,lozybean/rbenv,dark5un/nodenv,jmatbastos/rbenv,jeongdeoksu/rbenv,cehoffman/luaenv,peterpih/rbenv,chenfengyuan/pyenv,rridhan/rbenv,rowhit/pyenv,viclim/rbenv,VShangxiao/pyenv,jasonkarns/rbenv,sanemat/rbenv,mediadoneright/rbenv,janusnic/rbenv,syndbg/goenv,peterpih/rbenv,stivenrgalvis/rbenv,ox/rbenv,sheerun/rbenv,RinSer/rbenv,chenfengyuan/pyenv,Angeldude/rbenv,adarshmuthappa/nodenv,dongguangming/pyenv,nodenv/nodenv,jasonkarns/rbenv,jeremy/rbenv,avdhoweb/rbenv,UCASREN/pyenv,xshwy/pyenv,sstephenson/rbenv,ffdc-scm/rbenv-x86_64-linux,janusnic/rbenv,rridhan/rbenv,yyuu/pyenv,blueyed/rbenv,docwhat/rbenv,genome-vendor/pyenv,obimod/pyenv,sstephenson/rbenv,Ferocia/rbenv,rowhit/pyenv,jmatbastos/rbenv,recruit-mtl/pyenv,RinSer/rbenv,jmatbastos/rbenv,yogeshkakde/rbenv,pumpkineater/rbenv,cehoffman/luaenv,genome-vendor/pyenv,sebhahn/pyenv,syndbg/goenv,ffdc-scm/rbenv-x86_64-linux,princeofdarkness76/rbenv,jmatbastos/rbenv,josegonzalez/rbenv,dongguangming/pyenv,jeongdeoksu/rbenv,ffdc-scm/rbenv-x86_64-linux,lozybean/rbenv,SamuraiT/rbenv,OiNutter/nodenv,dark5un/nodenv,peterhoeg/rbenv,cehoffman/rbenv,ffdc-scm/rbenv-x86_64-linux,stivenrgalvis/rbenv,jmatbastos/rbenv,bradynpoulsen/rbenv,sstephenson/rbenv,reyesyang/rbenv,beck/rbenv,rinrinne/ndkenv,fly19890211/pyenv,recruit-mtl/pyenv,cyril/rbenv,michael-gabenna/rbenv,yogeshkakde/rbenv,viclim/rbenv,jeremy/rbenv,avdhoweb/rbenv,michael-gabenna/rbenv,yeonsh/pyenv,andrew-christianson/rbenv,janusnic/rbenv,asaaki/rsenv,Angeldude/rbenv,yyyc514/rbenv,yyyc514/rbenv,josegonzalez/rbenv,habinmusic/pyenv,syndbg/goenv,jf/rbenv,jrolfs/nodenv,lozybean/pyenv,bradynpoulsen/rbenv,yyyc514/rbenv,pumpkineater/rbenv,peterpih/rbenv,beck/rbenv,viclim/rbenv,howtolearntocode/rbenv,obimod/pyenv,michael-gabenna/rbenv,pyenv/pyenv,nebhale/cfenv,bradynpoulsen/rbenv,Ferocia/rbenv,intfrr/rbenv,takluyver/pyenv,lozybean/pyenv,Angeldude/rbenv,UCASREN/pyenv,RinSer/rbenv,cehoffman/nodenv,sebhahn/pyenv,jmatbastos/rbenv,codesdk/rbenv,mediadoneright/rbenv,mediadoneright/rbenv,adarshmuthappa/nodenv,nodenv/nodenv,yogeshkakde/rbenv,OiNutter/nodenv,lozybean/rbenv,reyesyang/rbenv,codesdk/rbenv,cehoffman/pyenv,intfrr/rbenv,jasonkarns/rbenv,yyuu/pyenv,jeongdeoksu/rbenv,dnsd/pyenv,ffdc-scm/rbenv-x86_64-linux,rbenv/rbenv,cyril/rbenv,pyenv/pyenv,codesdk/rbenv,zmwangx/pyenv,SamuraiT/rbenv,SamuraiT/rbenv,syndbg/goenv,fly19890211/pyenv,xshwy/pyenv,rridhan/rbenv,rbenv/rbenv,pumpkineater/rbenv,beck/rbenv,jmatbastos/rbenv,cehoffman/rbenv,sebhahn/pyenv