docs(readme): fix underscore colours guide (#646)

## Description

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maintainers
  why we should accept this pull request. -->

According to [this answer](https://askubuntu.com/a/985386),
`\e[58:2:R:G:Bm` is considered a "rarely used incorrect format" for
truecolor.

I tested it on Windows Terminal with WSL2, and it didn’t work. After
changing the format to `\e[58::2:R:G:Bm`, which is noted as the
"commonly used standard way," it worked as expected.

The change I made is as follows:

```diff
- set -as terminal-overrides ',*:Setulc=\E[58::2::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m'
+ set -as terminal-overrides ',*:Setulc=\E[58::2::::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m'
```

I’d appreciate it if someone could test this on more platforms.

## Related Issue(s)

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  - Fixes #<issue_number>
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## Screenshots

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README.md

@@ -398,7 +398,7 @@ To have undercurls show up and in color, add the following to your
 # Undercurl
 set -g default-terminal "${TERM}"
 set -as terminal-overrides ',*:Smulx=\E[4::%p1%dm'  # undercurl support
-set -as terminal-overrides ',*:Setulc=\E[58::2::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m'  # underscore colours - needs tmux-3.0
+set -as terminal-overrides ',*:Setulc=\E[58::2::::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m'  # underscore colours - needs tmux-3.0
 ```
 
 </details>