Correct the name of an optional tool: vobsubocr.

removeads.py

@@ -28,7 +28,7 @@ from shutil import copyfile, which
 def checkRequiredTools():
     logger = logging.getLogger(__name__)
     required = ['ffmpeg', 'ffprobe', 'mkvmerge']
-    optional = ['mkvextract', 'vobusbocr']
+    optional = ['mkvextract', 'vobsubocr']
     for tool in required:
         if which(tool) == None:
             logger.error('Required tool: %s is missing.' % tool)