docs: allow copy for install command (antfu-collective#204)

Author: hms5232

README.md

@@ -6,9 +6,9 @@
 
 <br>
 
-<pre>
-npm i -g <b>@antfu/ni</b>
-</pre>
+```
+npm i -g @antfu/ni
+```
 
 <a href='https://docs.npmjs.com/cli/v6/commands/npm'>npm</a> · <a href='https://yarnpkg.com'>yarn</a> · <a href='https://pnpm.io/'>pnpm</a> · <a href='https://bun.sh/'>bun</a>