SECURITY.md: update go versions

Change-Id: I59446965b198767af60d617da29b5c2e457829ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/197477
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

SECURITY.md

@@ -2,7 +2,7 @@
 
 ## Supported Versions
 
-We support the past two Go releases (for example, Go 1.11.x and Go 1.12.x).
+We support the past two Go releases (for example, Go 1.12.x and Go 1.13.x).
 
 See https://golang.org/wiki/Go-Release-Cycle and in particular the
 [Release Maintenance](https://github.com/golang/go/wiki/Go-Release-Cycle#release-maintenance)