Fixed two small spelling mistakes

- Saw these during normal usage today.
  "parens" or "parenthesis" was spelled as "parans".
  Fixed two instances of this to "parenthesis".

expand.cpp

@@ -1328,7 +1328,7 @@ static int expand_cmdsubst( parser_t &parser, const wcstring &input, std::vector
 		case -1:
 			parser.error( SYNTAX_ERROR,
                          -1,
-                         L"Mismatched parans" );
+                         L"Mismatched parenthesis" );
 			return 0;
 		case 0:
             outList.push_back(completion_t(input));