Update shimmer-firefly-governance-guide.md

[Phyloiota]

Grammar, spelling and Make the guide generic for IOTA and Shimmer

Description of change

Made the guide generic to both Firefly versions

Links to any relevant issues

Type of change

Choose a type of change, and delete any options that are not relevant.

• Documentation Enhancement

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own changes
• I have made sure that added/changed links still work
• I have commented my code, particularly in hard-to-understand areas

[Dr-Electron]

I fixed some broken links and now it looks good to me.
Keep in mind that now some links on the wiki will be changed with all those changes.

Also you might want to change the image (shimmer-gov.png)

[Phyloiota]

updated the images as suggested