message: Update src/doc/src/reference/unstable.md
diff: @@ -11,7 +11,7 @@ command-line flags. Options requiring this will be called out below.
  Anything which can be configured with a Z flag can also be set in the cargo
  config file (`.cargo/config.toml`) in the `unstable` table. For example:
  -```
  +```toml
  [unstable]
  mtime-on-use = 'yes'
  multitarget = 'yes'
  