docs/metrics: use underline-style heading markup

For consistency with the other docs. This also corrects the heading
levels, since level two was skipped.

Change-Id: Iab010cfca68af7d376944b0e8b2b8c076e33fbde

src/main/resources/Documentation/metrics.md

diff --git a/src/main/resources/Documentation/metrics.md b/src/main/resources/Documentation/metrics.md
index ef8b150..db11d23 100644
--- a/src/main/resources/Documentation/metrics.md
+++ b/src/main/resources/Documentation/metrics.md
@@ -1,4 +1,5 @@
-# Metrics
+Metrics
+=======
 
 Some metrics are emitted when replication occurs to a remote destination.
 The granularity of the metrics recorded is at destination level, however when a particular project replication is flagged
@@ -7,17 +8,21 @@
 The reason only slow metrics are published, rather than all, is to contain their number, which, on a big Gerrit installation
 could potentially be considerably big.
 
-### Project level
+Project level
+-------------
 
 * plugins_replication_latency_slower_than_<threshold>_<destinationName>_<ProjectName> - Time spent pushing <ProjectName> to remote <destinationName> (in ms)
 
-### Destination level
+Destination level
+-----------------
 
 * plugins_replication_replication_delay_<destinationName> - Time spent waiting before pushing to remote <destinationName> (in ms)
 * plugins_replication_replication_retries_<destinationName> - Number of retries when pushing to remote <destinationName>
 * plugins_replication_replication_latency_<destinationName> - Time spent pushing to remote <destinationName> (in ms)
 
-### Example
+Example
+-------
+
 ```
 # HELP plugins_replication_replication_delay_destination Generated from Dropwizard metric import (metric=plugins/replication/replication_delay/destination, type=com.codahale.metrics.Histogram)
 # TYPE plugins_replication_replication_delay_destination summary