webp-lossless-bitstream-spec: fix num_code_lengths check
in the 'Normal Code Length Code' description the number of valid code
lengths is 19, not 18.
@@ -910,7 +910,7 @@ First, `num_code_lengths` specifies the number of code lengths.
int num_code_lengths = 4 + ReadBits(4);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-If `num_code_lengths` is > 18, the bitstream is invalid.
+If `num_code_lengths` is > 19, the bitstream is invalid.
The code lengths are themselves encoded using prefix codes: lower level code
lengths, `code_length_code_lengths`, first have to be read. The rest of those
