Fix grammar for real numbers

Writing 22e-3 was not interpreted as a real and produced an error
message.

2e-3 and 22.e-3 were correctly interpreted.

src/language/PEGGrammar.hpp

@@ -46,7 +46,7 @@ struct real
                      >,
                 opt< exponent >
                 >,
-           seq< plus< digit, exponent > >
+           seq< plus< digit > , exponent >
          >{};
 
 struct escaped_c : one< '\'', '"', '?', '\\', 'a', 'b', 'f', 'n', 'r', 't', 'v' > {};