net/mail: Fix typo
net/mail: Fix typo

Fixes #21089

Change-Id: Idd65c7185b3e19f33958eb165cb5b09c06db3d56
Reviewed-on: https://go-review.googlesource.com/50110
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

codestation/go,CAFxX/go,vsekhar/elastic-go,cloudflare/go,cloudflare/go,akutz/go,nawawi/go,codestation/go,cloudflare/go,Cofyc/go,cloudflare/go,golang/go,nawawi/go,akutz/go,akutz/go,golang/go,vsekhar/elastic-go,vsekhar/elastic-go,codestation/go,nawawi/go,cloudflare/go,nawawi/go,josharian/go,vsekhar/elastic-go,polyverse-security/go,akutz/go,CAFxX/go,josharian/go,Cofyc/go,CAFxX/go,nawawi/go,cloudflare/go,codestation/go,golang/go,josharian/go,CAFxX/go,Cofyc/go,nawawi/go,CAFxX/go,polyverse-security/go,Cofyc/go,Cofyc/go,CAFxX/go,codestation/go,akutz/go,golang/go,codestation/go,golang/go,josharian/go,golang/go,codestation/go,nawawi/go,Cofyc/go,josharian/go,vsekhar/elastic-go,polyverse-security/go,polyverse-security/go,akutz/go,golang/go,vsekhar/elastic-go,golang/go,polyverse-security/go,polyverse-security/go,CAFxX/go,codestation/go,josharian/go,vsekhar/elastic-go,vsekhar/elastic-go,cloudflare/go,Cofyc/go,Cofyc/go,cloudflare/go,akutz/go,akutz/go,polyverse-security/go,polyverse-security/go,nawawi/go,christopher-henderson/Go,josharian/go,CAFxX/go,josharian/go