Fix broken sphinx link in CMake.rst.

Reviewers: delcypher, beanz

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69325

Author: kitbarton

llvm/docs/CMake.rst

@@ -519,7 +519,7 @@ LLVM-specific variables
 **SPHINX_EXECUTABLE**:STRING
   The path to the ``sphinx-build`` executable detected by CMake.
   For installation instructions, see
-  http://www.sphinx-doc.org/en/latest/install.html
+  http://www.sphinx-doc.org/en/latest/usage/installation.html
 
 **SPHINX_OUTPUT_HTML**:BOOL
   If enabled (and ``LLVM_ENABLE_SPHINX`` is enabled) then the targets for