Update add_convention.md (#190)

fix missing `

Author: ggqshr

docs/basic_facade_builder/add_convention.md

@@ -14,7 +14,7 @@ using add_direct_convention = basic_facade_builder</* see below */>;
 The alias templates `add_convention`, `add_indirect_convention`, and `add_direct_convention` of `basic_facade_builder<Cs, Rs, C>` add convention types to the template parameters. The expression inside `requires` is equivalent to `sizeof...(Os) > 0u` and each type in `Os` meets the [*ProOverload* requirements](../ProOverload.md). Let `F` be a facade type,
 
 - `add_convention` is equivalent to `add_indirect_convention`.
-- `add_indirect_convention` merges an implementation-defined convention type `IC` into Cs`, where:
+- `add_indirect_convention` merges an implementation-defined convention type `IC` into `Cs`, where:
   - `IC::is_direct` is `false`.
   - `typename IC::dispatch_type` is `D`.
   - `typename IC::overload_types` is a [tuple-like](https://en.cppreference.com/w/cpp/utility/tuple/tuple-like) type of distinct types in `Os`.