update icon: tmux (plain, plain-wordmark)

[weh]

Double check these details before you open a PR

• PR does not match another non-stale PR currently opened
• PR name matches the format new icon: Icon name (versions separated by comma). More details here
• PR's base is the develop branch.
• Your icons are inside a folder as seen here
• SVG matches the standards laid out here
• A new object is added in the devicon.json file at the correct alphabetic position as seen here

**This PR closes **

Link to prove your SVG is correct and up-to-date.

https://github.com/tmux/tmux/tree/master/logo

Fixes the following draft-release issue:

🔧 tmux1

Icon Folder

Issues:

• ❌ More than 1 path found in font SVG: plain
• ❌ plain in versions.font must have exactly 1 path and 1 fill color max. Found 2 paths and 0 fills.
• ❌ More than 1 path found in font SVG: plain-wordmark
• ❌ plain-wordmark in versions.font must have exactly 1 path and 1 fill color max. Found 2 paths and 0 fills.
• ⚠️ plain has no fill color, expected #1bb91f.
• ⚠️ plain-wordmark has no fill color, expected #1bb91f.