Various improvements to the ExtractingPartitions guide

[Pinguin2001]

• Remove unnecessary step to boot into fastbootd (you can get the current active slot in fastboot as well)
• Rename twrp.img => surfaceduo1-twrp.img (TWRPs filename got changed in the repo thus it needs to be adjusted in the guide as well)
• Use getvar current-slot instead of getvar all

[Pinguin2001]

Thanks! I will improve the guide accordingly

[Pinguin2001]

I have made improvements to the guide, it should be much clearer now.

[gus33000]

Thanks a lot! <3

[halal-beef]

https://github.com/WOA-Project/SurfaceDuo-Guides/pull/51/commits/73a07a43e6f460c78c14982aa6deba5079dbfaa0#diff-ed451b99c78a4671aa638113f6cfa09742069b4797e9bc959a24d04965dd7efeR48 on line 48 there's a typo btw