src/clapp/sub_parser.cpp: changed some methods/funcs to be easier to read

src/clapp/sub_parser.cpp

@@ -41,6 +41,7 @@ clapp::parser::basic_sub_parser_t::basic_sub_parser_t(
 }
 
 std::string clapp::parser::basic_sub_parser_t::gen_short_line_prefix() const {
-    return parent_parser.gen_short_line_prefix() + " " + sub_parser_name +
-           gen_short_line();
+    const std::string short_line_prefix{parent_parser.gen_short_line_prefix()};
+    const std::string short_line{gen_short_line()};
+    return short_line_prefix + " " + sub_parser_name + short_line;
 }