docs(README.md): fix 0pdd badge that didn't show up.

Use HTTP instead of HTTPS for an image to make it visible again.

This is a workaround for yegor256/0pdd#289

Correction for 1512041 commit.

Fix #1398

[skip ci]

README.md

@@ -1,7 +1,7 @@
 # My Stamps
 
 [![Build Status](https://travis-ci.org/php-coder/mystamps.svg?branch=master)](https://travis-ci.org/php-coder/mystamps)
-[![PDD Status](https://www.0pdd.com/svg?name=php-coder/mystamps)](https://www.0pdd.com/p?name=php-coder/mystamps)
+[![PDD Status](http://www.0pdd.com/svg?name=php-coder/mystamps)](https://www.0pdd.com/p?name=php-coder/mystamps)
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=ru.mystamps%3Amystamps&metric=alert_status)](https://sonarcloud.io/dashboard?id=ru.mystamps%3Amystamps)
 [![Uptime Statistic](https://badgen.net/uptime-robot/month/ur243278-551fbb732949dbdee27c7552)](https://stats.uptimerobot.com/1jXAjFpgP)