runtime: fix incorrect comment
runtime: fix incorrect comment

This comment is probably a hold-over from when the heap bitmap was
interleaved and the shift was 0, 2, 4, or 6. Now the shift is 0, 1, 2,
or 3.

Change-Id: I096ec729e1ca31b708455c98b573dd961d16aaee
Reviewed-on: https://go-review.googlesource.com/18531
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Rick Hudson <838bd060584c511f21c75c69ea04b7b120d86af0@golang.org>

Endika/go,Samurais/go,Samurais/go,ganboing/go-esx,cloudflare/go,sndnvaps/go,nawawi/go,4ad/go,binarycrusader/go,polyverse-security/go,polyverse-security/go,goodlang/good,mwhudson/go,cloudflare/go,vanloswang/go,cyberroadie/go,cyberroadie/go,derekmarcotte/go,suncycheng/go,momchil-velikov/go,linux-on-ibm-z/go,cloudflare/go,4ad/go,mwhudson/go,Endika/go,ImJasonH/go,Endika/go,Samurais/go,nawawi/go,cloudflare/go,golang/go,josharian/go,vanloswang/go,goodlang/good,josharian/go,sndnvaps/go,ganboing/go-esx,suncycheng/go,codestation/go,Endika/go,codestation/go,ganboing/go-esx,4ad/go,nawawi/go,vanloswang/go,polyverse-security/go,binarycrusader/go,Cofyc/go,miolini/go,linux-on-ibm-z/go,ImJasonH/go,Endika/go,theif519/go,sndnvaps/go,4ad/go,vsekhar/elastic-go,sndnvaps/go,theif519/go,momchil-velikov/go,mwhudson/go,miolini/go,cyberroadie/go,ganboing/go-esx,ImJasonH/go,golang/go,binarycrusader/go,Cofyc/go,Cofyc/go,goodlang/good,cyberroadie/go,ImJasonH/go,vsekhar/elastic-go,cyberroadie/go,ganboing/go-esx,sndnvaps/go,momchil-velikov/go,derekmarcotte/go,theif519/go,goodlang/good,binarycrusader/go,4ad/go,Samurais/go,derekmarcotte/go,akutz/go,binarycrusader/go,golang/go,mwhudson/go,derekmarcotte/go,theif519/go,cyberroadie/go,4ad/go,polyverse-security/go,miolini/go,akutz/go,suncycheng/go,linux-on-ibm-z/go,4ad/go,Samurais/go,4ad/go,codestation/go,polyverse-security/go,cloudflare/go,CAFxX/go,Cofyc/go,nawawi/go,golang/go,nawawi/go,golang/go,CAFxX/go,vsekhar/elastic-go,cyberroadie/go,akutz/go,vsekhar/elastic-go,cloudflare/go,derekmarcotte/go,ImJasonH/go,CAFxX/go,binarycrusader/go,theif519/go,theif519/go,nawawi/go,momchil-velikov/go,vsekhar/elastic-go,Cofyc/go,Samurais/go,ImJasonH/go,Cofyc/go,momchil-velikov/go,vanloswang/go,ganboing/go-esx,sndnvaps/go,nawawi/go,momchil-velikov/go,josharian/go,vsekhar/elastic-go,cyberroadie/go,mwhudson/go,akutz/go,cloudflare/go,josharian/go,derekmarcotte/go,golang/go,CAFxX/go,polyverse-security/go,akutz/go,Samurais/go,suncycheng/go,codestation/go,josharian/go,polyverse-security/go,miolini/go,vanloswang/go,josharian/go,ImJasonH/go,cloudflare/go,Cofyc/go,mwhudson/go,suncycheng/go,binarycrusader/go,miolini/go,vanloswang/go,theif519/go,linux-on-ibm-z/go,mwhudson/go,akutz/go,theif519/go,linux-on-ibm-z/go,ImJasonH/go,miolini/go,goodlang/good,akutz/go,momchil-velikov/go,derekmarcotte/go,suncycheng/go,suncycheng/go,linux-on-ibm-z/go,CAFxX/go,christopher-henderson/Go,CAFxX/go,akutz/go,ganboing/go-esx,linux-on-ibm-z/go,CAFxX/go,miolini/go,sndnvaps/go,suncycheng/go,Samurais/go,Endika/go,goodlang/good,vsekhar/elastic-go,linux-on-ibm-z/go,goodlang/good,polyverse-security/go,codestation/go,Endika/go,derekmarcotte/go,momchil-velikov/go,vanloswang/go,codestation/go,codestation/go,miolini/go,sndnvaps/go,ganboing/go-esx,Cofyc/go,CAFxX/go,nawawi/go,codestation/go,vanloswang/go,mwhudson/go,goodlang/good,vsekhar/elastic-go,golang/go,josharian/go,Endika/go,josharian/go,golang/go,binarycrusader/go