docs: iio: Correct a typo

There are two EP9312, one of them should be 9315
Signed-off-by: NYanteng Si <siyanteng@loongson.cn>
Link: https://lore.kernel.org/r/20210119030320.2860870-1-siyanteng@loongson.cnSigned-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/iio/ep93xx_adc.rst

@@ -13,7 +13,7 @@ touchscreen/ADC module.
 ====================
 
 Numbering scheme for channels 0..4 is defined in EP9301 and EP9302 datasheets.
-EP9307, EP9312 and EP9312 have 3 channels more (total 8), but the numbering is
+EP9307, EP9312 and EP9315 have 3 channels more (total 8), but the numbering is
 not defined. So the last three are numbered randomly, let's say.
 
 Assuming ep93xx_adc is IIO device0, you'd find the following entries under