docs: Update installation instructions

Change-Id: Id3639b4ac1110d2f770e89aa9f24d716f49d7b6d
diff --git a/docs/redmine_issue.py b/docs/redmine_issue.py
index 35c3b12..87be49d 100644
--- a/docs/redmine_issue.py
+++ b/docs/redmine_issue.py
@@ -50,7 +50,7 @@
         base = app.config.redmine_project_url
         if not base:
             raise AttributeError
-    except AttributeError, err:
+    except AttributeError as err:
         raise ValueError('redmine_project_url configuration value is not set (%s)' % str(err))
     #
     slash = '/' if base[-1] != '/' else ''