Add last missing SecureHeaders namespace fix for OPT_OUT

jacobbednarz · jacobbednarz · commit 90ac658bc8eb · 2017-09-25T12:00:57.000+10:00
I missed this in #361 but didn't notice until _after_ it was merged.
diff --git a/upgrading-to-4-0.md b/upgrading-to-4-0.md
@@ -15,7 +15,7 @@ config.cookies = {
 }
 
 # nuclear option, just make things work again
-config.cookies = OPT_OUT
+config.cookies = SecureHeaders::OPT_OUT
 ```
 
 ## script_src must be set

Original file line number	Diff line number	Diff line change
`@@ -15,7 +15,7 @@ config.cookies = {`
`15`	`15`	`}`
`16`	`16`
`17`	`17`	`# nuclear option, just make things work again`
`18`		`-config.cookies = OPT_OUT`
	`18`	`+config.cookies = SecureHeaders::OPT_OUT`
`19`	`19`	```
`20`	`20`
`21`	`21`	`## script_src must be set`