Fix typo in sam require

tts/sam/index.js

@@ -11,7 +11,7 @@ module.exports = class extends BaseEngine {
         return 'en';
     }
     async getSpeechFile(text, filepath, voice = this.getDefaultVoice(), params = {}) {
-        let sam = new samjs();
+        let sam = new Sam();
         const buf = sam.buf8(text);
         const file = new wavefile.WaveFile();
         file.fromScratch(1, 22050, 8, buf);