Fix comment directive to prevent webpage display

`..` immediately following paragraph text at the same indentation is
interpreted as a continuation of that text, as seen on the website:

https://docs.godotengine.org/en/stable/contributing/how_to_contribute.html#technical-contributions

> Upload your plugins to the Godot Asset Library to make them available to others. .. update to talk about Asset Store later

I feel the intent was still clear, but it does look a bit weird at the moment.

Further information:
 - There must be a blank line between the text block and the comment for the
   comment to be properly interpreted as a comment.
 - There must not be two blank lines between the two list items, once the
   comment is ignored. Otherwise, the generated output produces two separate
   lists. This a perceptibly different amount of padding compared to when they
   were structurally part of the same list in HTML output, since Godot adds
   extra padding around `<ul>` tags.

contributing/how_to_contribute.rst

@@ -74,8 +74,9 @@ Technical contributions
 - **Write Plugins (GDScript, C#, & more)**
   Community addons are not directly included in the core engine download or repository, yet they provide essential quality of life upgrades for your fellow game developers.
   Upload your plugins to the `Godot Asset Library <https://godotengine.org/asset-library/asset>`_ to make them available to others.
-  .. update to talk about Asset Store later
 
+  ..
+    update to talk about Asset Store later
 - **Demo projects (GDScript, C#, and making Assets)**
   We provide new users with `demo projects <https://github.com/godotengine/godot-demo-projects/>`_ so they can quickly test new features or get familiar with the engine in the first place.
   At industry events, we might even exhibit these demo projects to showcase what Godot can do!