Fix gdt command: require address argument (#2497)

Before this commit, the `gdt` command incorrectly did not require the `address` argument, while it was required.

Because of that, the command crashed like this:

```
pwndbg> gdt
'gdt': Decode X86-64 GDT entries at address

See also:
* https://wiki.osdev.org/Global_Descriptor_Table
* https://wiki.osdev.org/GDT_Tutorial

Note:
In 64-bit mode, the Base and Limit values are ignored, each descriptor covers the entire linear address space regardless of what they are set to.
Exception occurred: gdt: int() argument must be a string, a bytes-like object or a real number, not 'NoneType' (<class 'TypeError'>)
For more info invoke `set exception-verbose on` and rerun the command
or debug it by yourself with `set exception-debugger on`
```

pwndbg/commands/gdt.py

@@ -24,7 +24,6 @@ In 64-bit mode, the Base and Limit values are ignored, each descriptor covers th
 parser.add_argument(
     "address",
     type=int,
-    nargs="?",
     help="x86-64 GDTR base address (e.g. read from sgdt instruction from [16:79] bits)",
 )