Use a non-greedy match for the double-dollar-sign escaping

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

convert.rb

@@ -28,7 +28,7 @@ class KatexRenderer
     rendered = content.gsub /\\\(((?:[^\\]|\\[^\)])*)\\\)/ do |match|
       render(false, $~[1])
     end
-    rendered = rendered.gsub /\$\$((?:[^\$]|$[^\$])*)\$\$/ do |match|
+    rendered = rendered.gsub /\$\$(.*?)\$\$/ do |match|
       render(true, $~[1])
     end
     return rendered