fix(gentoo): fix tab color completion (#9810)

This commit fixes an issue in the gentoo theme introduced in 3bb5e977.
Due to incorrect line width calculations, colors are incorrectly applied
when using tab.

Fixes #9778
Signed-off-by: NThomas De Backer <mosterdt@debacker.me>

themes/gentoo.zsh-theme

@@ -25,4 +25,4 @@ gentoo_precmd() {
 autoload -U add-zsh-hook
 add-zsh-hook precmd gentoo_precmd
 
-PROMPT='%(!.%B%F{red}.%B%F{green}%n@)%m %F{blue}%(!.%1~.%~) ${vcs_info_msg_0_}%(!.#.$)%{$reset_color%} '
+PROMPT='%(!.%B%F{red}.%B%F{green}%n@)%m %F{blue}%(!.%1~.%~) ${vcs_info_msg_0_}%(!.#.$)%k%b%f '