Build the parser's syntax error prefix with one concat() (#6013)

Since #6004 made the "while parsing" part unconditional, the three appends
at the start of exception_message() are one fixed string around the
context. Review follow-up to #6004.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

single_include/nlohmann/json.hpp

@@ -23802,11 +23802,7 @@ class parser
 
     std::string exception_message(const token_type expected, const std::string& context)
     {
-        std::string error_msg = "syntax error ";
-
-        error_msg += concat("while parsing ", context, ' ');
-
-        error_msg += "- ";
+        std::string error_msg = concat("syntax error while parsing ", context, " - ");
 
         if (last_token == token_type::parse_error)
         {