docs: remove mention to .pde files

We don't support them anymore.

docs/config/default

@@ -687,7 +687,7 @@ INPUT_ENCODING         = UTF-8
 # *.hxx *.hpp *.h++ *.idl *.odl *.cs *.php *.php3 *.inc *.m *.mm *.dox *.py
 # *.f90 *.f *.for *.vhd *.vhdl
 
-FILE_PATTERNS          = *.c *.cpp *.ino *.pde *.h
+FILE_PATTERNS          = *.c *.cpp *.ino *.h
 
 # The RECURSIVE tag can be used to turn specify whether or not subdirectories
 # should be searched for input files as well. Possible values are YES and NO.