Fix grammar error in TSX section (#554)

Signed-off-by: Connor Shea <connor.james.shea@gmail.com>

Author: connorshea

src/docs/guide/usage/transformer/typescript.md

@@ -73,7 +73,7 @@ const result = transform("lib.ts", sourceCode, {
 
 ## TSX
 
-Transforming TSX files are supported as well.
+Transforming TSX files is supported as well.
 See [JSX transform](./jsx) for more information.
 
 ## Rewriting import extensions