doc: strconv.QuoteToGraphic in go1.6.txt

Change-Id: I2f05d0b62deb5d7d0886f6fc5af5e7b79792efba
Reviewed-on: https://go-review.googlesource.com/14381Reviewed-by: Rob Pike <r@golang.org>

doc/go1.6.txt

@@ -8,4 +8,5 @@ NaCl is no longer restricted to pepper_41 (https://golang.org/cl/13958/)
 
 API additions and behavior changes:
 
+strconv: QuoteTOGraphic (https://golang.org/cl/14184/)
 text/template: ExecError (https://golang.org/cl/13957/)