Fix missing closing bold tag in html.md (#6546)

Author: tommyp

usage-rules/html.md

@@ -6,7 +6,7 @@
 - **Always** add unique DOM IDs to key elements (like forms, buttons, etc) when writing templates, these IDs can later be used in tests (`<.form for={@form} id="product-form">`)
 - For "app wide" template imports, you can import/alias into the `my_app_web.ex`'s `html_helpers` block, so they will be available to all LiveViews, LiveComponent's, and all modules that do `use MyAppWeb, :html` (replace "my_app" by the actual app name)
 
-- Elixir supports `if/else` but **does NOT support `if/else if` or `if/elsif`. **Never use `else if` or `elseif` in Elixir**, **always** use `cond` or `case` for multiple conditionals.
+- Elixir supports `if/else` but **does NOT support `if/else if` or `if/elsif`**. **Never use `else if` or `elseif` in Elixir**, **always** use `cond` or `case` for multiple conditionals.
 
   **Never do this (invalid)**: