update sphinx cmake

cmake/FindSphinx.cmake

@@ -72,7 +72,7 @@ function( Sphinx_add_target target_name builder conf cache source destination )
     ${source}
     ${destination}
     COMMENT "Generating sphinx documentation: ${builder}"
-    COMMAND cd ${destination} && ln -s ./index_*.html index.html
+    COMMAND cd ${destination} && ln -sf ./index_*.html index.html
     )
 
   set_property(