Use backticks not vertical bars to denote variables in comments for /modules/audio_processing

Bug: webrtc:12338
Change-Id: I85bff694dd2ead83c939c4d1945eff82e1296001
No-Presubmit: True
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/227161
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34690}

modules/audio_processing/transient/dyadic_decimator_unittest.cc

@@ -42,7 +42,7 @@ TEST(DyadicDecimatorTest, DyadicDecimateErrorValues) {
                                static_cast<int16_t*>(NULL), kOutBufferLength);
   EXPECT_EQ(0u, out_samples);
 
-  // Less than required |out_length|.
+  // Less than required `out_length`.
   out_samples = DyadicDecimate(test_buffer_even_len, kEvenBufferLength,
                                false,  // Even sequence.
                                test_buffer_out, 2);