Use singular noun (#1259)

Author: chenxsan

docs/getting-started/index.mdx

@@ -109,7 +109,7 @@ things like the following markup:
 JSX is good for **components**.
 It makes repeating things more clear and allows for separation of concerns.
 MDX fully supports [JSX syntax][jsx].
-Any line that start with the `<` characters starts a JSX block.
+Any line that start with the `<` character starts a JSX block.
 
 ### MDX