Fix formatting in stubs documentation for execution speed description

Author: rcalicdan

docs/advanced/stubs.md

@@ -231,4 +231,4 @@ Calling `$repo->calculateScore(-5, 'sports')` will be validated and rejected by
 
 1. **Vendor Isolation is Preserved**: Un-stubbed vendor files in `vendor/` remain completely ignored by `FileFilter`, protecting your application from third-party DocBlock bleed.
 2. **Liskov Substitution Principle (LSP)**: Child classes extending stubbed vendor classes or implementing stubbed vendor interfaces automatically inherit the stub's clean contracts.
-3. **$O(1)$ Execution Speed**: Stubs are parsed once during boot/cache warm-up into `StubManager` in static memory, adding zero disk I/O to runtime method calls.
+3. **O(1) Execution Speed**: Stubs are parsed once during boot/cache warm-up into `StubManager` in static memory, adding zero disk I/O to runtime method calls.