[docs] Fix broken flink-training links in hands-on section

Author: kylerballiet

docs/layouts/shortcodes/training_link.html

@@ -23,6 +23,6 @@
         - file: The absolute path to the image file (required)
         - name: The rendered link name (required)
 */}}
-<a href="{{ .Site.Params.TrainingExercises }}/blob/{{ .Site.Params.Branch }}/{{ .Get "file" }}">
+<a href="{{ .Site.Params.TrainingExercises }}/tree/{{ .Site.Params.TrainingBranch | default "master" }}/{{ .Get "file" }}">
     {{ .Get "name" }}
 </a>

docs/layouts/shortcodes/training_repo.html

@@ -23,7 +23,7 @@
         - file: The absolute path to the image file (required)
         - name: The rendered link name (required)
 */}}
-<a href="{{ .Site.Params.TrainingExercises }}/tree/{{ .Site.Params.Branch }}/">
+<a href="{{ .Site.Params.TrainingExercises }}/tree/{{ .Site.Params.TrainingBranch | default "master" }}/">
     flink-training-repo
 </a>