Change docs links to actual file extension (.md)

See https://gerrit-review.googlesource.com/83950 for detailed context.

Change-Id: I674c1f6eab6ed28956fbb685c83c88f87ac015f5

src/main/resources/Documentation/about.md

diff --git a/src/main/resources/Documentation/about.md b/src/main/resources/Documentation/about.md
index a711d86..f104962 100644
--- a/src/main/resources/Documentation/about.md
+++ b/src/main/resources/Documentation/about.md
@@ -8,4 +8,4 @@
 
 For this secondary index synchronization to work, http must be enabled in both
 instances and the plugin must be configured with valid credentials. For further
-information, refer to [config](config.html) documentation.
+information, refer to [config](config.md) documentation.