all: use cannot instead of can not

You can not use cannot, but you cannot spell cannot can not.

Change-Id: I2f0971481a460804de96fd8c9e46a9cc62a3fc5b
Reviewed-on: https://go-review.googlesource.com/19772
Reviewed-by: Rob Pike <r@golang.org>

src/database/sql/sql.go

@@ -199,7 +199,7 @@ type Scanner interface {
 	//    time.Time
 	//    nil - for NULL values
 	//
-	// An error should be returned if the value can not be stored
+	// An error should be returned if the value cannot be stored
 	// without loss of information.
 	Scan(src interface{}) error
 }