database/sql: fix Conn's doc typo

Fixes #21798

Change-Id: Ided31a8b22c220acdeb3938cac41ce8db9a110c3
Reviewed-on: https://go-review.googlesource.com/62290Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/database/sql/sql.go

@@ -1493,7 +1493,7 @@ func (db *DB) Conn(ctx context.Context) (*Conn, error) {
 
 type releaseConn func(error)
 
-// Conn represents a single database session rather a pool of database
+// Conn represents a single database session rather than a pool of database
 // sessions. Prefer running queries from DB unless there is a specific
 // need for a continuous single database session.
 //