nixos/nixpkgs/docs: fix typo in assertOneOf example

This example was confusing at first because the element the message
indicated wasn't in the list of possible values was but the possible
values didn't match up either.  This ensures the example is consistent
with the logic being presented.

doc/functions/library/asserts.xml

@@ -103,7 +103,7 @@ stderr> assert failed
    <title>Ensuring a user provided a possible value</title>
 <programlisting><![CDATA[
 let sslLibrary = "bearssl";
-in lib.asserts.assertOneOf "sslLibrary" sslLibrary [ "openssl" "bearssl" ];
+in lib.asserts.assertOneOf "sslLibrary" sslLibrary [ "openssl" "libressl" ];
 => false
 stderr> trace: sslLibrary must be one of "openssl", "libressl", but is: "bearssl"
         ]]></programlisting>