Remove json flag from afu_synth_setup example
The following example doesn't work with the --json flag, not supported by afu_synth_setup.
@@ -114,7 +114,7 @@ The software side is contained entirely in [sw/hello_afu.c](sw/hello_afu.c):
`README` file:
```console
- $ afu_synth_setup --source hw/sources.txt --json hw/hello_afu.json build_synth
+ $ afu_synth_setup --source hw/sources.txt build_synth
$ cd build_synth
$ ${OPAE_PLATFORM_ROOT}/bin/run.sh
```
