sql: fix a typo and update wording

Via: pingcap/docs-cn#917

Author: lilin90

sql/character-set-support.md

@@ -73,12 +73,12 @@ The collation names in TiDB follow these conventions:
     | Suffix | Meaning            |
     |:-------|:-------------------|
     | \_ai   | Accent insensitive |
-    | \_as   | Accent insensitive |
+    | \_as   | Accent sensitive   |
     | \_ci   | Case insensitive   |
     | \_cs   | Case sensitive     |
     | \_bin  | Binary             |
 
-> **Note**: For now, TiDB supports on some of the collations in the above table.
+> **Note**: Currently, TiDB only supports some of the collations in the above table.
 
 ## Database character set and collation