doc/articles/laws_of_reflection.html: fix name of variable

Thanks to c.emil.hessman@gmail.com for pointing out the error.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12572043

doc/articles/laws_of_reflection.html

@@ -148,7 +148,7 @@ Continuing, we can do this:
 {{code "/doc/progs/interface.go" `/var empty interface{}/` `/STOP/`}}
 
 <p>
-and our empty interface value <code>e</code> will again contain
+and our empty interface value, <code>empty</code>, will again contain
 that same pair, (<code>tty</code>, <code>*os.File</code>). That's
 handy: an empty interface can hold any value and contains all the
 information we could ever need about that value.