Update Color3DLUT description

src/PIL/ImageFilter.py

@@ -377,7 +377,7 @@ class Color3DLUT(MultibandFilter):
                   tuples with floats. Channels are changed first,
                   then first dimension, then second, then third.
                   Value 0.0 corresponds lowest value of output, 1.0 highest.
-    :param channels: Number of channels in the table. Could be 3 or 4.
+    :param channels: Number of channels in the table. Should be 3 or 4.
                      Default is 3.
     :param target_mode: A mode for the result image. Should have not less
                         than ``channels`` channels. Default is ``None``,

src/libImaging/ColorLUT.c

@@ -83,7 +83,7 @@ ImagingColorLUT3D_linear(
     ImagingSectionCookie cookie;
 
     if (table_channels < 3 || table_channels > 4) {
-        PyErr_SetString(PyExc_ValueError, "table_channels could be 3 or 4");
+        PyErr_SetString(PyExc_ValueError, "table_channels should be 3 or 4");
         return NULL;
     }