encoding/gob: fix comment

Fixes #4100.

R=adg, r, dave
CC=golang-dev
https://golang.org/cl/6535055

src/pkg/encoding/gob/doc.go

@@ -118,7 +118,7 @@ elements using the standard gob encoding for their type, recursively.
 
 Maps are sent as an unsigned count followed by that many key, element
 pairs. Empty but non-nil maps are sent, so if the sender has allocated
-a map, the receiver will allocate a map even no elements are
+a map, the receiver will allocate a map even if no elements are
 transmitted.
 
 Structs are sent as a sequence of (field number, field value) pairs.  The field