Update README.md

Co-authored-by: Jordan Harband <[email protected]>

README.md

@@ -73,7 +73,7 @@ options can be:
 * `opts.string` - a string or array of strings argument names to always treat as
 strings
 * `opts.boolean` - A boolean, string, or array of strings to always treat as
-booleans. If `true` will treat all double hyphenated arguments without equal signs
+booleans. If `true` will treat all double-hyphenated arguments without equal signs
 as boolean (e.g. affects `--foo`, not `-f` or `--foo=bar`) A boolean option will
 consume the following argument if it is the string `true` or `false`. (e.g. `--foo false`)
 * `opts.alias` - an object mapping string names to strings or arrays of string