Update link to TSC charter issue (#161)

The README currently mentions that we're seeking charter and points to
<nodejs/TSC#175>, but that issue has been
closed for some time. A newer, more urgent-sounding issue is now up at
<nodejs/TSC#486>.

README.md

@@ -3,7 +3,7 @@
 # Security Working Group
 
 ***Note: this group is in the process of seeking Charter by the TSC
-(https://github.com/nodejs/TSC/issues/175)***
+(https://github.com/nodejs/TSC/issues/486)***
 
 ## Mandate