Fix "class names" in Basics long concept text (exercism#671)

concepts/basic-syntax/about.md

@@ -189,7 +189,7 @@ $sum = add(1, 2);
 
 ## Naming conventions
 
-Classnames should all be `PascalCase`.
+Class names should all be `PascalCase`.
 Depending on the style standard; variables, functions, and method names may be either `camelCase` or `snake_case`.
 Names may contain letters `a-zA-Z`, numbers `0-9`, and underscores `_` but they cannot start with a number.