docs: Fix spelling in no-callback-literal.md (#135)

Author: jtenclay

docs/rules/no-callback-literal.md

@@ -6,7 +6,7 @@ When invoking a callback function which uses the Node.js error-first callback pa
 
 ## 📖 Rule Details
 
-When a function is named `cb` or `callback`, then it must be invoked with a first argument that is `undefined`, `null`, an `Error` class, or a subclass or `Error`.
+When a function is named `cb` or `callback`, then it must be invoked with a first argument that is `undefined`, `null`, an `Error` class, or a subclass of `Error`.
 
 Examples of 👎 **incorrect** code for this rule: