fix dependency table in readme

Author: egpbos

README.md

@@ -48,7 +48,7 @@ make install
 
 | `xtensor-fftw` | `xtensor` | `xtl`   | `fftw`  |
 |----------------|-----------|---------|---------|
-|  master        | ^0.20.10  |  ^0.6.7 | ^3.3.8  |
+|  master        | ^0.21.2   |  ^0.6.9 | ^3.3.8  |
 
 ## Usage