Fix loading of CSS file

Remove the leading '/' from the css file path, because otherwise
Gerrit fails to load this file:

Failed to load resource: the server responded with a status of 404
https://<host>/plugins/reviewers/static/reviewers//reviewers.css

Change-Id: Ib8269a52096c6ae6df26ba3697cb9a52de6533b1
Signed-off-by: Edwin Kempin <ekempin@google.com>
diff --git a/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersForm.gwt.xml b/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersForm.gwt.xml
index 8a5430d..1ea6083 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersForm.gwt.xml
+++ b/src/main/java/com/googlesource/gerrit/plugins/reviewers/ReviewersForm.gwt.xml
@@ -27,5 +27,5 @@
   <!-- added in order to make this plugin as simple as possible   -->
   <!-- Specify the app entry point class.                         -->
   <entry-point class="com.googlesource.gerrit.plugins.reviewers.client.ReviewersPlugin"/>
-  <stylesheet src="/reviewers.css"/>
+  <stylesheet src="reviewers.css"/>
 </module>