bytes: correct Map documentation
bytes: correct Map documentation

Fix incorrect reference to string instead of byte slice.

Change-Id: I95553da32acfbcf5dde9613b07ea38408cb31ae8
Reviewed-on: https://go-review.googlesource.com/68090
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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