Clarify that you can pass a directory to `odin build`

Changes the usage information to this:
```
D:\Software\odin\odin.exe is a tool for managing Odin source code
Usage:
        D:\Software\odin\odin.exe command [arguments]
Commands:
        build     compile .odin file, or directory of .odin files, as an executable.
                  one must contain the program's entry point, all must be in the same package.
        run       same as 'build', but also then runs the newly compiled executable.
        check     parse and type check .odin file
        query     parse, type check, and output a .json file containing information about the program
        docs      generate documentation for a .odin file
        version   print version
```

src/main.cpp

@@ -160,8 +160,9 @@ void usage(String argv0) {
 	print_usage_line(0, "Usage:");
 	print_usage_line(1, "%.*s command [arguments]", LIT(argv0));
 	print_usage_line(0, "Commands:");
-	print_usage_line(1, "build     compile .odin file as executable");
-	print_usage_line(1, "run       compile and run .odin file");
+	print_usage_line(1, "build     compile .odin file, or directory of .odin files, as an executable.");
+	print_usage_line(1, "          one must contain the program's entry point, all must be in the same package.");
+	print_usage_line(1, "run       same as 'build', but also then runs the newly compiled executable.");
 	print_usage_line(1, "check     parse and type check .odin file");
 	print_usage_line(1, "query     parse, type check, and output a .json file containing information about the program");
 	print_usage_line(1, "docs      generate documentation for a .odin file");