strconv: fix documentation for CanBackquote. Space is not a control character.
strconv: fix documentation for CanBackquote.
Space is not a control character.

Fixes issue 8571.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/137380043

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go