fix: Grammar issues

Author: adinhodovic

sox/transform.py

@@ -345,7 +345,7 @@ def _validate_output_format(self, output_format):
             raise ValueError('channels must be a positive number')
 
         if not isinstance(bitrate, float) and bitrate is not None:
-            raise ValueError('bitrate must be an float or None')
+            raise ValueError('bitrate must be a positive float or None')
 
         if encoding not in ENCODING_VALS + [None]:
             raise ValueError(