Minor commandline_flags.md fix

Change 1KB -> 512B, which is the default behavior:

```
$ man radare2 | grep -i malloc://
       -           Equivalent of 'r2 malloc://512'
```

src/first_steps/commandline_flags.md

@@ -82,7 +82,7 @@ Open a file in write mode and do not parse the headers (raw mode).
 $ r2 -nw file
 ```
 
-Quickly get into the r2 shell opening a 1KB malloc virtual file, handy for testing things. note that a single dash is an alias for malloc://1024
+Quickly get into the r2 shell opening a 512B malloc virtual file, handy for testing things. note that a single dash is an alias for malloc://512
 
 ```console
 $ r2 -