Document the actual Sphinx executable variable

The installation instructions referred to a Sphinx cache variable that
does not exist, so setting it had no effect.

Name the variable used by the Sphinx find module.

Change-Id: I72c6a6fc1c6cbc1f8a2602644dbe20929c9862cd

docs/source/installation.rst

diff --git a/docs/source/installation.rst b/docs/source/installation.rst
index 9511f5e..44ef910 100644
--- a/docs/source/installation.rst
+++ b/docs/source/installation.rst
@@ -762,7 +762,7 @@
      ``SuiteSparse_<COMPONENT>_INCLUDE_DIR`` and
      ``SuiteSparse_<COMPONENT>_LIBRARY`` (for components ``AMD``, ``CAMD``,
      ``CCOLAMD``, ``CHOLMOD``, ``COLAMD``, ``SPQR``, ``Config``).
-   - **Sphinx**: ``Sphinx_EXECUTABLE``.
+   - **Sphinx**: ``Sphinx_BUILD_EXECUTABLE``.
 
 .. NOTE::