README: Fix non-existent iconv in macOS instruction (#19475)

Follows-up 22e444c5c7 (GH-18670). There is currently no formula called
"iconv". [1][2]

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

There package is called "libiconv". [3]

Once installed, `./configure` still fails due to a discovery issue.

```
$ ./configure --enable-debug
…
checking for libiconv... no
configure: error: Please specify the install prefix of iconv with --with-iconv=<DIR>
```

In 2020, as part of the switch from Intel to ARM, Homebrew adopted /opt
as the standard location instead of /usr/local. [4][5]

Rather than complicating the README with a mandatory `--with-iconv`
path (or --without-iconv) for macOS users, improve the discovery
to support Homebrew's new location.

[1]: https://brew.sh/
[2]: https://github.com/Homebrew/homebrew-core/.
[3]: https://formulae.brew.sh/formula/libiconv
[4]: https://apple.stackexchange.com/a/437622/33762
[5]: https://docs.brew.sh/FAQ#why-is-the-default-installation-prefix-opthomebrew-on-apple-silicon

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`: