Update README.md

Author: lemire

README.md

@@ -191,7 +191,7 @@ the command line help.
 
 You may directly download automatically generated single-header files:
 
-https://github.com/fastfloat/fast_float/releases/download/v1.1.2/fast_float.h
+https://github.com/fastfloat/fast_float/releases/download/v3.4.0/fast_float.h
 
 ## Credit