update readme [skip ci]

Author: takenori-y

README.md

@@ -20,7 +20,7 @@
 
 ## Documentation
 
-- See [this page](https://sp-nitech.github.io/diffsptk/3.1.0/) for the reference manual.
+- See [this page](https://sp-nitech.github.io/diffsptk/3.2.0/) for the reference manual.
 - Our [paper](https://www.isca-speech.org/archive/ssw_2023/yoshimura23_ssw.html) is available on the ISCA Archive.
 
 ## Installation