docs: Fix link to Blackfriday documentation (#551)

exampleSite/config.toml

@@ -38,7 +38,7 @@ enableGitInfo = false
   section = [ "HTML", "RSS" ]
 
 # Configure BlackFriday Markdown rendering.
-#   See: https://gohugo.io/readfiles/bfconfig/
+#   See: https://gohugo.io/getting-started/configuration/#configure-blackfriday
 [blackfriday]
   hrefTargetBlank = true  # `true` opens external links in a new tab.
   fractions = true  # `false` disables smart fractions (e.g. 5/12 formatted as a fraction).