[web] Fixed container download links.

web/content/releases/6.1.0.md

 +++
 date = "2018-03-13T11:00:13+01:00"
 title = " OpenGeoSys 6.1.0"
+tag = "6.1.0"
 author = "Lars Bilke"
 release_date = "2019-03-20"
 

web/content/releases/6.2.0.md

 +++
 date = "2019-03-13T11:00:13+01:00"
 title = "OpenGeoSys 6.2.0"
+tag = "6.2.0"
 author = "Lars Bilke"
 release_date = "2019-03-20"
 

web/content/releases/6.2.1.md

 +++
 date = "2019-08-13T11:00:13+01:00"
 title = "OpenGeoSys 6.2.1"
+tag = "6.2.1"
 author = "Lars Bilke"
 release_date = "2019-08-13"
 

web/content/releases/6.2.2.md

 +++
 date = "2020-01-06T11:00:13+01:00"
 title = "OpenGeoSys 6.2.2"
+tag = "6.2.2"
 author = "Lars Bilke"
 release_date = "2020-01-04"
 

web/layouts/partials/releases-single.html

 {{ $gh_release := print "https://github.com/ufz/ogs/releases/tag/" .Title }}
-{{ $gh_download := print "https://github.com/ufz/ogs/releases/download/" .Title }}
-{{ $file_prefix := print "ogs-" .Title }}
+{{ $gh_download := print "https://github.com/ufz/ogs/releases/download/" .Params.tag }}
+{{ $file_prefix := print "ogs-" .Params.tag }}
 <p>Released on {{ dateFormat "Monday, Jan 2, 2006" .Params.release_date }}, <a href="{{ $gh_release }}"><i class="fab fa-github"></i> GitHub release</a></p>
 {{ if .Params.downloads }}
   <div class="note">