Updating brew install command to avoid pkg-config warning (#891)

* Updating brew install command to avoid warning

I am using Mac OS Catalina 10.15.7.
To avoid getting the following warning : 
$ configure: WARNING: pkg-config is missing. Some dependencies may not be detected correctly.
$ brew install pkg-config 
needs to be run.

* Allowing build of additional modules

I was getting the following message:
$ The necessary bits to build these optional modules were not found:
_hashlib              _ssl      
Updating the command line with:
$ brew install [email protected]    
resolved the issue.

* making this PR only about removing warning

Author: izumiberat

setup.rst

@@ -385,7 +385,7 @@ for the header and library files to your ``configure`` command.  For example,
 
 with **Homebrew**::
 
-    $ brew install openssl xz gdbm
+    $ brew install pkg-config openssl xz gdbm
 
 and ``configure`` Python versions >= 3.7::