fix functions table

Author: JPryce-Aklundh

modules/ROOT/pages/functions/index.adoc

@@ -208,7 +208,7 @@ LOAD CSV functions can be used to get information about the file that is process
 
 
 [[header-query-functions-logarithmic]]
-**xref::functions/mathematical-logarithmic.adoc[Logarithmic functions]**
+== Logarithmic functions
 
 These functions all operate on numerical expressions only, and will return an error if used on any other values.