build_testbench: Fix mhwaveedit command

The command `mhWaveEdit audio_out.wav` was incorrect because the 
package installs the binary as `mhwaveedit` (all lowercase). This 
commit corrects the capitalization to avoid command not found errors.

Signed-off-by: Suraj Sonawane <surajsonawane0215@gmail.com>

Author: SurajSonawane2415

developer_guides/testbench/build_testbench.rst

@@ -115,7 +115,7 @@ it can be launched to an audio editor tool such as mhWaveEdit:
 .. code-block:: bash
 
    paplay audio_out.wav
-   mhWaveEdit audio_out.wav
+   mhwaveedit audio_out.wav
 
 .. figure:: fig_mhwaveedit.png