Update configuration.md

Very minor grammar issue.

docs/user-guide/configuration.md

@@ -10,7 +10,7 @@ Project settings are configured by default using a YAML configuration file in
 the project directory named `mkdocs.yml`. You can specify another path for it
 by using the `-f`/`--config-file` option (see `mkdocs build --help`).
 
-As a minimum, this configuration file must contain the `site_name`. All other settings are optional.
+At a minimum, this configuration file must contain the `site_name`. All other settings are optional.
 
 ## Project information