[DOC] Replace removed method in example for OpenSSL::Config#to_s

[hoshi-sano]

OpenSSL::Config#[]= is used in OpenSSL::Config#to_s example,

openssl/ext/openssl/ossl_config.c

Lines 308 to 313 in 8367b16

	* Given the following configuration being created:
	*
	* config = OpenSSL::Config.new
	* #=> #<OpenSSL::Config sections=[]>
	* config['default'] = {"foo"=>"bar","baz"=>"buz"}
	* #=> {"foo"=>"bar", "baz"=>"buz"}

even though it was removed by #342 .

Other two methods are removed because the corresponding functions
disappeared in OpenSSL 1.1.0.

• OpenSSL::Config#add_value
• OpenSSL::Config#[]=

irb(main):001> RUBY_VERSION
=> "3.3.5"
irb(main):002> OpenSSL::VERSION
=> "3.2.0"
irb(main):003> OpenSSL::OPENSSL_VERSION
=> "OpenSSL 3.0.2 15 Mar 2022"
irb(main):004> config = OpenSSL::Config.new
=> #<OpenSSL::Config sections=[]>
irb(main):005> config['default'] = {"foo"=>"bar","baz"=>"buz"}
(irb):5:in `<main>': undefined method `[]=' for an instance of OpenSSL::Config (NoMethodError)

[rhenium]

Good catch. Thank you!