Spelling fix: change "an" to "a"

Change-Id: I1eb3811ef083f1fbad5605cad1a0d3af559cd332

av1/encoder/rdopt.c

diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 694d38e..b899087 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -808,7 +808,7 @@
     // rdcosts close to each other), the intra-coded candidate was subjectively
     // observed to be a bit less blurry, with a corresponding increase in
     // SSIMULACRA 2 scores.
-    // Apply an 1.125x inter block bias to increase overall perceptual
+    // Apply a 1.125x inter block bias to increase overall perceptual
     // compression efficiency, while still allowing the encoder to pick inter
     // prediction when it's beneficial.
     rd_cost->dist += rd_cost->dist >> 3;