synt/misc [FIX]: let randomseries mention rngtype in usage

src/synt/misc/randomseries.cc

@@ -56,6 +56,7 @@ int main(int iargc, char* argv[])
     "usage: randomseries filename [-v] [-t type] [-o]" "\n"
     "                    [-nseries n] [-nsamples n] [-dt v]" "\n"
     "                    [-std v] [-mean v] [-seed n]" "\n"
+    "                    [-rngtype rng]" "\n"
     "   or: randomseries --help|-h" "\n"
     "   or: randomseries --xhelp[=type]\n"
   };
@@ -79,6 +80,7 @@ int main(int iargc, char* argv[])
     "-nseries n   produce 'n' time series\n"
     "-nsamples n  produce 'n' samples per series\n"
     "-dt v        set sampling interval to 'v' seconds\n"
+    "-rngtype rng select random number generator\n"
     "-seed n      initialize random number generator with 'n'\n"
     "-std v       set standard deviation to 'v'\n"
     "-mean v      set mean value of samples to 'v'\n"