fix(@angular/cli): Update README.md project generation text. (#5958)

angular-cli/packages/@angular/cli/utilities/validate-project-name.ts looks for hypen(-) rather than the listed underscore(_).

Author: killrazor

README.md

@@ -50,8 +50,8 @@ ng help
 ### Generating and serving an Angular project via a development server
 
 ```bash
-ng new PROJECT_NAME
-cd PROJECT_NAME
+ng new PROJECT-NAME
+cd PROJECT-NAME
 ng serve
 ```
 Navigate to `http://localhost:4200/`. The app will automatically reload if you change any of the source files.