usb: Documentation: gadget-testing: fix parameter for capture channel mask

Fix the UAC2 parameter capture channel mask
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/usb/gadget-testing.txt

@@ -629,7 +629,7 @@ Function-specific configfs interface
 The function name to use when creating the function directory is "uac2".
 The uac2 function provides these attributes in its function directory:
 
-	chmask - capture channel mask
+	c_chmask - capture channel mask
 	c_srate - capture sampling rate
 	c_ssize - capture sample size (bytes)
 	p_chmask - playback channel mask