Depluralise tools in the success case

[chshersh]

It's a very common case when only a single tool is synced/installed:

• tool install xyz: install a new single tool
• tool sync xyz: sync only one tool from the config
• Only 1 tool is specified in the config (or left after filtering all errors)

But the success message always uses the plural form tools.

I propose to use tool instead of tools when only 1 tool is installed:

https://github.com/chshersh/tool-sync/blob/dd7a006da995285eb925965472d67f87107d8c19/src/sync.rs#L84