strconv: fix documentation for CanBackquote.

Space is not a control character.

Fixes #8571.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/137380043

src/strconv/quote.go

@@ -143,7 +143,7 @@ func AppendQuoteRuneToASCII(dst []byte, r rune) []byte {
 
 // CanBackquote reports whether the string s can be represented
 // unchanged as a single-line backquoted string without control
-// characters other than space and tab.
+// characters other than tab.
 func CanBackquote(s string) bool {
 	for len(s) > 0 {
 		r, wid := utf8.DecodeRuneInString(s)