[skip ci] README: Fix non-existent iconv in macOS instruction

Follows-up 22e444c (GH-18670). There is currently no formula called
"iconv" at https://brew.sh/ or https://github.com/Homebrew/homebrew-core/.

```
$ brew install iconv
Warning: No available formula with the name "iconv". Did you mean icon or cconv?
```

There is one called libiconv, however.

https://formulae.brew.sh/formula/libiconv

Author: Krinkle

README.md

@@ -55,7 +55,7 @@ sudo dnf install re2c bison autoconf make libtool ccache libxml2-devel sqlite-de
 On MacOS, you can install these using `brew`:
 
 ```shell
-brew install autoconf bison re2c iconv libxml2 sqlite
+brew install autoconf bison re2c libiconv libxml2 sqlite
 ```
 
 or with `MacPorts`: