chore: make migration bold

Author: mdevils

README.md

@@ -14,7 +14,7 @@ css-selector-parser
   * `latest`: refers to `selectors-4`
   * `progressive`: `latest` + accepts unknown psudo-classes, psudo-elements and attribute case sensitivity modifiers
 
-*Important:* [Migrating from 1.x](CHANGELOG.md#220).
+**Important:** [Migrating from 1.x](CHANGELOG.md#220).
 
 Installation
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