modif typo in print_usage message

src/param.c

@@ -20,7 +20,7 @@ void print_usage() {
     "<-pdbDir>          PDB directory containing the name of the receptor/ligand .pdb\n"
     "<-rec>             name of the receptor\n"
     "<-lig>             name of the ligand\n"
-    "OPTIONNAL\n"
+    "OPTIONAL\n"
     "=========\n\n"
     "<-outputPDB>       Output directory for the constructed PDB (ligand)\n"
     "                   If nothing is specified, there will be no PDB output\n"