Change URL for Medium in header and footer buttons

The URL for the SPIFFE blog has changed. The content of the blog has
been refreshed. This change is per Umair Khan of HPE.

Signed-off-by: Steve Anderson <steve.anderson@hpe.com>

config.toml

@@ -27,7 +27,7 @@ spire = "**SPIRE**, the SPIFFE Runtime Environment, is an extensible system that
 [params.socialmedia]
 twitter       = "SPIFFEio"
 slack         = "https://slack.spiffe.io/"
-medium        = "https://scytale.io/blog/"
+medium        = "https://blog.spiffe.io/"
 github        = "https://github.com/spiffe"
 stackoverflow = "https://stackoverflow.com/questions/tagged/spiffe"