Improve documentation of cli flag --project (palantir#3703)

The current documentation of the cli flag `--project` is a little confusing as to which the path of a file or the path of a directory can be specified.

As both are valid options, updating the documentation to make it clear that either the path to a `tsconfig.json` file or the path to a directory containing a `tsconfig.json` file can be specified.

src/tslint-cli.ts

@@ -174,9 +174,10 @@ const options: Option[] = [
         type: "string",
         describe: "tsconfig.json file",
         description: dedent`
-            The path or directory containing a tsconfig.json file that will be
-            used to determine which files will be linted. This flag also enables
-            rules that require the type checker.`,
+            The path to the tsconfig.json file or to the directory containing
+            the tsconfig.json file. The file will be used to determine which
+            files will be linted. This flag also enables rules that require the
+            type checker.`,
     },
     {
         name: "type-check",