Linkify the SpecAugment paper reference

docs/waveform_transforms/band_stop_filter.md

@@ -3,7 +3,7 @@
 _Added in v0.21.0_
 
 Apply band-stop filtering to the input audio. Also known as notch filter or
-band reject filter. It relates to the frequency mask idea in the SpecAugment paper.
+band reject filter. It relates to the frequency mask idea in the [SpecAugment paper :octicons-link-external-16:](https://arxiv.org/abs/1904.08779).
 Center frequency gets picked in mel space, so it is somewhat aligned with human hearing,
 which is not linear. Filter steepness (6/12/18... dB / octave) is parametrized. Can also
 be set for zero-phase filtering (will result in a 6 dB drop at cutoffs).