doc/transforms: avoid lua lexer error in luaxform example

Pygments 2.21 lexes "x < 0 or x > y" in Lua as a Lua 5.4 variable
attribute (e.g. "<const>"), producing an error token on the "0". Sphinx
reports this as a highlighting failure warning, and as the PDF build
treats warnings as errors, "make dist" fails while building the PDF.

Reorder the comparisons so the Lua example is lexed correctly. The logic
of the example is unchanged.

doc/userguide/rules/transforms.rst

@@ -418,12 +418,12 @@ the buffer.
            end
        end
        local str_len = #input
-       if offset < 0 or offset > str_len then
+       if offset < 0 or str_len < offset then
            print("offset is out of bounds: " .. offset)
            return nil
        end
        str_len = str_len - offset
-       if bytes < 0 or bytes > str_len then
+       if bytes < 0 or str_len < bytes then
            print("invalid bytes " ..  bytes .. " or bytes > length " .. bytes .. " length " .. str_len)
            return nil
        end