Fix whitespace bug in deprecation template

This fixes a bug in the template for the deprecation documentation
which caused extra blank lines to appear. Those extra blank lines
caused linting job failures, so this removes them.

data/deprecations/templates/_deprecation_template.md.erb

@@ -15,7 +15,7 @@ Do not edit this page directly.
 To add a deprecation, use the example.yml file in `/data/deprecations/templates` as a template,
 then run `bin/rake gitlab:docs:compile_deprecations`.
 -->
-<% if milestones.any? %>
+<% if milestones.any? -%>
   <%- milestones.each do |milestone| %>
 ## <%= milestone %>
     <%- deprecations.select{|d| d["removal_milestone"] == milestone}.each do |deprecation| %>