net: fix typo in comment

Change-Id: I8b2063e65d3454a694a789c1682dacfe0bea2e19
Reviewed-on: https://go-review.googlesource.com/8965Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/fd_plan9.go

@@ -11,7 +11,7 @@ import (
 	"time"
 )
 
-// Network file descritor.
+// Network file descriptor.
 type netFD struct {
 	// locking/lifetime of sysfd + serialize access to Read and Write methods
 	fdmu fdMutex