Change http to https for security links (#5950)

Signed-off-by: Nguyen Van Trung <trungnv@vn.fujitsu.com>

Mirrored from https://github.com/envoyproxy/envoy @ 5313bbc56e63500b48cd1c207f686cd5a9f68acf

CONTRIBUTING.md

@@ -33,7 +33,7 @@ Or to use a hermetic Docker container:
 ```
 
 This process builds RST documentation directly from the proto files, merges it with the static RST
-files, and then runs [Sphinx](http://www.sphinx-doc.org/en/stable/rest.html) over the entire tree to
+files, and then runs [Sphinx](https://www.sphinx-doc.org/en/stable/rest.html) over the entire tree to
 produce the final documentation. The generated RST files are not committed as they are regenerated
 every time the documentation is built.