database/sql/driver: fix typo

Change-Id: I6e7035db4b3e2a09e5655eb7646eea9d99fb7118
Reviewed-on: https://go-review.googlesource.com/c/162917
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/driver/driver.go

@@ -137,7 +137,7 @@ type Pinger interface {
 
 // Execer is an optional interface that may be implemented by a Conn.
 //
-// If a Conn implements neither ExecerContext nor Execer Execer,
+// If a Conn implements neither ExecerContext nor Execer,
 // the sql package's DB.Exec will first prepare a query, execute the statement,
 // and then close the statement.
 //