keywords: add LATERAL (pingcap#21447)

Author: hfxsd

keywords.md

@@ -393,6 +393,7 @@ TiDB 从 v7.5.3 和 v7.6.0 开始提供 [`INFORMATION_SCHEMA.KEYWORDS`](/informa
 - LAST_BACKUP
 - LAST_VALUE (R-Window)
 - LASTVAL
+- LATERAL
 - LEAD (R-Window)
 - LEADING (R)
 - LEAVE (R)