[Doc] Add default boundary file in default ini file

doc/default_files/richards-parameters.xml

@@ -91,6 +91,7 @@ adding an empty line, make text **bold** or ``monospaced``.
   <category name="boundary">
     <parameter name="file">
       <definition> Path to the boundary condition file. </definition>
+      <suggestion> 2d_infiltr.bcdat </suggestion>
       <values> path </values>
     </parameter>