fix: LoroText.length is a property not a function (#24)

Author: cobbweb

pages/docs/tutorial/text.mdx

@@ -237,7 +237,7 @@ Get the character at the given position.
 Delete and return the string at the given range and insert a string at the same
 position.
 
-### `length(): number`
+### `length: number`
 
 Get the length of text