Use same style as zsh documentation

[a1346054]

• [x] I provide my work under the project license.
• [x] I grant such license of my work as is required for the purposes of future print editions to Ben Straub and Scott Chacon.

Changes

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 \$

Context

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

[ben]

LGTM, thanks!