cmd/cgo: correct comment on Package.rewriteCall
cmd/cgo: correct comment on Package.rewriteCall

Account for changes in https://golang.org/cl/31233.

Change-Id: I3311c6850a3c714d18209fdff500dd817e9dfcb2
Reviewed-on: https://go-review.googlesource.com/31594
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

CAFxX/go,polyverse-security/go,vsekhar/elastic-go,polyverse-security/go,goodlang/good,derekmarcotte/go,suncycheng/go,christopher-henderson/Go,ImJasonH/go,cloudflare/go,josharian/go,vsekhar/elastic-go,linux-on-ibm-z/go,nawawi/go,suncycheng/go,suncycheng/go,CAFxX/go,polyverse-security/go,vsekhar/elastic-go,goodlang/good,derekmarcotte/go,josharian/go,josharian/go,Cofyc/go,codestation/go,golang/go,codestation/go,golang/go,goodlang/good,polyverse-security/go,suncycheng/go,goodlang/good,akutz/go,nawawi/go,akutz/go,ImJasonH/go,derekmarcotte/go,goodlang/good,cloudflare/go,ImJasonH/go,josharian/go,codestation/go,codestation/go,CAFxX/go,suncycheng/go,linux-on-ibm-z/go,codestation/go,polyverse-security/go,CAFxX/go,CAFxX/go,ImJasonH/go,josharian/go,golang/go,linux-on-ibm-z/go,Cofyc/go,akutz/go,derekmarcotte/go,ImJasonH/go,Cofyc/go,nawawi/go,vsekhar/elastic-go,akutz/go,polyverse-security/go,ImJasonH/go,derekmarcotte/go,linux-on-ibm-z/go,polyverse-security/go,josharian/go,derekmarcotte/go,cloudflare/go,josharian/go,linux-on-ibm-z/go,ImJasonH/go,vsekhar/elastic-go,cloudflare/go,golang/go,vsekhar/elastic-go,golang/go,nawawi/go,linux-on-ibm-z/go,golang/go,goodlang/good,linux-on-ibm-z/go,josharian/go,Cofyc/go,cloudflare/go,CAFxX/go,golang/go,Cofyc/go,akutz/go,akutz/go,vsekhar/elastic-go,Cofyc/go,vsekhar/elastic-go,suncycheng/go,codestation/go,golang/go,nawawi/go,derekmarcotte/go,akutz/go,Cofyc/go,cloudflare/go,codestation/go,cloudflare/go,nawawi/go,goodlang/good,akutz/go,linux-on-ibm-z/go,CAFxX/go,polyverse-security/go,nawawi/go,suncycheng/go,nawawi/go,ImJasonH/go,cloudflare/go,Cofyc/go,goodlang/good,codestation/go,derekmarcotte/go,CAFxX/go,suncycheng/go