Fix broken link to PSR-18 in index

index.rst refers to a link to PSR-18, but does not actually provide that
link.

Let's:

- move the link to PSR-18 to the epilog so that both discovery.rst and
   index.rst link to PSR-18 properly

- change the link to PSR-17 to HTTPS, because it's 2021 :p

Author: Xenonym

conf.py

@@ -306,7 +306,8 @@ def setup(app):
 #texinfo_no_detailmenu = False
 
 rst_epilog = """
-.. _PSR-7: http://www.php-fig.org/psr/psr-7
+.. _PSR-7: https://www.php-fig.org/psr/psr-7
+.. _PSR-18: https://www.php-fig.org/psr/psr-18
 .. _Composer: https://getcomposer.org
 .. _HttplugBundle: https://github.com/php-http/HttplugBundle
 """

discovery.rst

@@ -291,4 +291,3 @@ and that it is not used by the Discovery Service by default. It is simply
 provided as a convenient option when writing tests.
 
 .. _PSR-17: http://www.php-fig.org/psr/psr-17
-.. _PSR-18: http://www.php-fig.org/psr/psr-18