Use same style as zsh documentation

Both the zsh manpage, and the documentation at:
https://zsh.sourceforge.io/Doc/Release/User-Contributions.html#Version-Control-Information
which is linked in the progit book talk about single quoted '${vcs_info_msg_0_}'
instead of trying to escape the \$

Their syntax is less confusing if anyone ends up searching for more
information in the documentation.

Author: Your Name

book/A-git-in-other-environments/sections/zsh.asc

@@ -28,8 +28,8 @@ autoload -Uz vcs_info
 precmd_vcs_info() { vcs_info }
 precmd_functions+=( precmd_vcs_info )
 setopt prompt_subst
-RPROMPT=\$vcs_info_msg_0_
-# PROMPT=\$vcs_info_msg_0_'%# '
+RPROMPT='${vcs_info_msg_0_}'
+# PROMPT='${vcs_info_msg_0_}%# '
 zstyle ':vcs_info:git:*' formats '%b'
 ----