Fix mattermost URL

docker/2.0/README.md

@@ -1,7 +1,7 @@
 Mattermost
 ==========
 
-http:/mattermost.org
+http://mattermost.org
 
 Mattermost is an open-source team communication service. It brings team messaging and file sharing into one place, accessible across PCs and phones, with archiving and search.
 

docker/2.1/README.md

@@ -1,7 +1,7 @@
 Mattermost
 ==========
 
-http:/mattermost.org
+http://mattermost.org
 
 Mattermost is an open-source team communication service. It brings team messaging and file sharing into one place, accessible across PCs and phones, with archiving and search.
 

docker/dev/README.md

@@ -1,7 +1,7 @@
 Mattermost
 ==========
 
-http:/mattermost.org
+http://mattermost.org
 
 Mattermost is an open-source team communication service. It brings team messaging and file sharing into one place, accessible across PCs and phones, with archiving and search.