playground.html: filesystem is not a word.

R=adg, rsc
CC=golang-dev
https://golang.org/cl/2370043

doc/playground.html

@@ -13,7 +13,7 @@ There are limitations to the programs that can be run in the Playground.
 They must be single-threaded (but they may use many goroutines).
 There are also limits on execution time, and CPU and memory usage.
 The Playground can access only a subset of the standard library
-(notably absent are network and filesystem access).
+(notably absent are network and file system access).
 Therefore, the only communication a Playground program has to the outside world
 is via standard output.
 </div>