doc/Doxyfile: use 5 columns for alphabetical index

This reverts a hunk from 671005558a
Reduces diff to what we use on the server
Suggested-by: Timothy Gu
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/Doxyfile

@@ -759,7 +759,7 @@ ALPHABETICAL_INDEX     = YES
 # the COLS_IN_ALPHA_INDEX tag can be used to specify the number of columns
 # in which this list will be split (can be a number in the range [1..20])
 
-COLS_IN_ALPHA_INDEX    = 2
+COLS_IN_ALPHA_INDEX    = 5
 
 # In case all classes in a project start with a common prefix, all
 # classes will be put under the same header in the alphabetical index.