Replace map! with foreach in printCommandLineHelp

Fixes compilation in release mode. `dub build --build=release` using DMD
2.062:

```.dub/packages/vibe-d/source/vibe/core/args.d(106): Error: function
vibe.core.args.printCommandLineHelp is a nested function and cannot be
accessed from std.array.join!(MapResult!(optionString, string[]),
string).join
.dub/packages/vibe-d/source/vibe/core/args.d(107): Error: function
vibe.core.args.printCommandLineHelp is a nested function and cannot be
accessed from std.array.join!(MapResult!(optionString, string[]),
string).join```