Corrected Volume Unit

if volume ends with 'db' change it to 'dB', else ffmpeg may produce error in some cases.

Author: SamarRizvi

src/PHPVideoToolkit/AudioFormat.php

@@ -373,7 +373,7 @@ public function setVolume($volume)
                 throw new \InvalidArgumentException('Unrecognised volume value "'.$volume.'" set in \\PHPVideoToolkit\\'.get_class($this).'::setVolume. The value must be higher than or equal to 0.');
             }
 
-            $this->_format['audio_volume'] = $volume;
+            $this->_format['audio_volume'] = str_replace('db', 'dB', $volume);
             return $this;
         }