Update protobuf -> text exposition format link (#1159)

This was moved around 2 weeks ago.  https://github.com/RichiH/OpenMetrics/commit/3592c278748628d838f062e92c79c837aa070641Signed-off-by: James Pearson Hughes <jamesp@wepay.com>

content/docs/instrumenting/exposition_formats.md

@@ -15,7 +15,7 @@ NOTE: Some earlier versions of Prometheus supported an exposition format based o
 addition to the current text-based format. As of version 2.0, however, Prometheus no
 longer supports the Protobuf-based format. You can read about the reasoning behind
 this change in [this
-document](https://github.com/RichiH/OpenMetrics/blob/master/protobuf_vs_text.md).
+document](https://github.com/RichiH/OpenMetrics/blob/master/markdown/protobuf_vs_text.md).
 
 ## Text-based format