update docstring

Author: Ji Chen

torchaudio/models/_wavernn.py

@@ -91,7 +91,7 @@ def forward(self, specgram: Tensor) -> Tensor:
 
 
 class _Stretch2d(nn.Module):
-    r"""Stretch layer upscales the frequency and time dimensions of spectrogram.
+    r"""Stretch layer upscales the frequency and time dimensions of a spectrogram.
 
     Args:
         time_scale: the scale factor in time dimension
@@ -125,7 +125,7 @@ def forward(self, specgram: Tensor) -> Tensor:
 
 
 class _UpsampleNetwork(nn.Module):
-    r"""Upsample block upscales the dimensions of spectrogram to match waveform.
+    r"""Upsample block upscales the dimensions of a spectrogram to match waveform.
 
     Args:
         upsample_scales: the list of upsample scales