playground.html: filesystem is not a word.

R=adg, rsc
CC=golang-dev
https://golang.org/cl/2370043
diff --git a/doc/playground.html b/doc/playground.html
index a1cdb70..01d3adc 100644
--- a/doc/playground.html
+++ b/doc/playground.html
@@ -13,7 +13,7 @@
 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>