Fix :::warning syntax in server-rendering docs (react-navigation#1423)

Removed extra space in :::warning syntax in docs/server-rendering.md to enable proper rendering as a highlighted markdown warning block.

Author: hehex9

versioned_docs/version-7.x/server-rendering.md

@@ -12,7 +12,7 @@ This guide will cover how to server render your React Native app using React Nat
 1. Rendering the correct layout depending on the request URL
 2. Setting appropriate page metadata based on the focused screen
 
-::: warning
+:::warning
 
 Server rendering support is currently limited. It's not possible to provide a seamless SSR experience due to a lack of APIs such as media queries. In addition, many third-party libraries often don't work well with server rendering.