cmd/vet: fix formatting of headings in doc.go.
cmd/vet: fix formatting of headings in doc.go.

This will cause godoc to correctly render these docs,
since go/doc.ToHTML requires no punctuation for headings.

Change-Id: Ic95245147d3585f2ccc59d4424fcab17d2a5617b
Reviewed-on: https://go-review.googlesource.com/31319
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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