[HDRP] Fix format issue in HDRP documentation

The hdrp documentation is badly formatted: https://docs.unity3d.com/Packages/[email protected]/manual/Authoring-LUTs.html

however we can see this error from .html file only, when looking with .md we can't spot the error.

This PR attempt to fix this format issue.

Author: sebastienlagarde

Packages/com.unity.render-pipelines.high-definition/Documentation~/Authoring-LUTs.md

@@ -1,6 +1,7 @@
 # Lookup textures
 
 You can use lookup textures (LUTs) in the High Definition Render Pipeline (HDRP) . To create a LUT, use external software. 
+
 |Page|Description|
 |-|-|
 |[Understand LUTs](rendering-luts-understand.md)|Learn about LUTs in HDRP.|