psnr.h,cosmetics: fix a typo (PNSR -> PSNR)

Change-Id: I2adea9f150852c106acc57e5aeeac571d6bd15fb

aom_dsp/psnr.h

diff --git a/aom_dsp/psnr.h b/aom_dsp/psnr.h
index 96a17f4..afe6e08 100644
--- a/aom_dsp/psnr.h
+++ b/aom_dsp/psnr.h
@@ -31,7 +31,7 @@
 
 /*!\brief Converts SSE to PSNR
  *
- * Converts sum of squared errros (SSE) to peak signal-to-noise ratio (PNSR).
+ * Converts sum of squared errros (SSE) to peak signal-to-noise ratio (PSNR).
  *
  * \param[in]    samples       Number of samples
  * \param[in]    peak          Max sample value