Rewrite credits for Butterworth.cpp

Author: sukibaby

README.md

@@ -86,7 +86,7 @@ Code from the following projects is included in this repository:
    - See  `lib/freetype/` for more info.
 - [lua](https://www.lua.org/license.html), using its GPLv3-compatible permissive license
    - See  `lib/lua/` for more info.
-- [libiir](https://src.lwithers.me.uk/c/libiir) (used in `src/Editor/Butterworth.cpp`), which is also GPLv3
+- The code in `src/Editor/Butterworth.cpp` is based upon [liir.c](https://www.exstrom.com/journal/sigproc/dsigproc.html) by Exstrom Labratories, which is GPL
 - The FFT routines in `/src/Editor/FFT.cpp` are based upon code from [Takuya OOURA's General Purpose FFT Package](https://www.kurims.kyoto-u.ac.jp/~ooura/fft.html), which is released freely