Added Catch::clara namespace to doc for adding Opt

- Thanks to sakamoto-poteko in #1159 for point out its ommission

docs/own-main.md

@@ -81,6 +81,7 @@ int main( int argc, char* argv[] )
   int height = 0; // Some user variable you want to be able to set
   
   // Build a new parser on top of Catch's
+  using namespace Catch::clara;
   auto cli 
     = session.cli() // Get Catch's composite command line parser
     | Opt( height, "height" ) // bind variable to a new option, with a hint string