[fix #34] fix link (http -> https)

Author: fmichonneau

_extras/data.md

@@ -23,7 +23,7 @@ with it using exactly the same tools we'll learn about today.
 >
 > The database is currently available in csv, json, and sqlite.
 >
-> This database is not designed for research as it intentionally removes some of the real-world complexities. The original database is published at [Ecological Archives](http://esapubs.org/archive/ecol/E090/118/) and this version of the database should be used for research purposes. The Python code used for converting the original database to this teach version is included as 'create_portal_teach_dataset.py'. Suggested changes or additions to this dataset can be requested or contributed in the project GitHub repository [https://github.com/weecology/portal-teachingdb](https://github.com/weecology/portal-teachingdb).
+> This database is not designed for research as it intentionally removes some of the real-world complexities. The original database is published at [Ecological Archives](https://esapubs.org/archive/ecol/E090/118/) and this version of the database should be used for research purposes. The Python code used for converting the original database to this teach version is included as 'create_portal_teach_dataset.py'. Suggested changes or additions to this dataset can be requested or contributed in the project GitHub repository [https://github.com/weecology/portal-teachingdb](https://github.com/weecology/portal-teachingdb).
 >
 > **CITATION:** Ernest, Morgan; Brown, James; Valone, Thomas; White, Ethan P. (2017): Portal Project Teaching Database. figshare. https://doi.org/10.6084/m9.figshare.1314459.v6
 {: .prereq}