[Docs] Bug fix for docs homepage

Removes reference to non-existent Reference Documentation page.

llvm-svn: 372032

Author: drichards-87

llvm/docs/index.rst

@@ -65,7 +65,6 @@ Getting Started, How-tos, Developer Guides, and Tutorials.
    UserGuides
    ProgrammingDocumentation
    SubsystemDocumentation
-   ReferenceDocumentation
 
 :doc:`UserGuides`
   For those new to the LLVM system.