Minor doc fix: PSNR (#196)

Summary:
correct reference to [`peak_signal_noise_ratio` functional](https://pytorch.org/torcheval/main/generated/torcheval.metrics.functional.peak_signal_noise_ratio.html) in the [`PSNR` class](https://pytorch.org/torcheval/main/generated/torcheval.metrics.PeakSignalNoiseRatio.html)

Pull Request resolved: #196

Test Plan: Is a documentation fix, does not need tests

Reviewed By: diego-urgell

Differential Revision: D56738555

Pulled By: JKSenthil

fbshipit-source-id: e4320d9db6bed44bbbd38bd1e84162e1a9d9f0e8

torcheval/metrics/image/psnr.py

@@ -26,7 +26,7 @@
 class PeakSignalNoiseRatio(Metric[torch.Tensor]):
     """
     Compute the PSNR (Peak Signal to Noise Ratio) between two images.
-    Its functional version is `torcheval.metrics.functional.psnr`
+    Its functional version is `torcheval.metrics.functional.peak_signal_noise_ratio`
 
     Args:
         data_range (float): the range of the input images. Default: None.