fix get_full_command for messages with caption

[dashedman]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context.

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A

@dispatcher.message_handler(commands=["command"], commands_ignore_caption=False, content_types=types.ContentType.ANY)
async def message_handler(message: types.Message):
    print( message.get_full_command() )    

Test Configuration:

• Operating System: Windows 10
• Python version: 3.8