doc: update output of example in mimeParams.set()

Actual output of example in `mimeParams.set()` is mismatched.

PR-URL: #49718
Reviewed-By: Matthew Aitken <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: deokjinkim

doc/api/util.md

@@ -1334,7 +1334,7 @@ const { params } = new MIMEType('text/plain;foo=0;bar=1');
 params.set('foo', 'def');
 params.set('baz', 'xyz');
 console.log(params.toString());
-// Prints: foo=def&bar=1&baz=xyz
+// Prints: foo=def;bar=1;baz=xyz
 ```
 
 ```cjs
@@ -1344,7 +1344,7 @@ const { params } = new MIMEType('text/plain;foo=0;bar=1');
 params.set('foo', 'def');
 params.set('baz', 'xyz');
 console.log(params.toString());
-// Prints: foo=def&bar=1&baz=xyz
+// Prints: foo=def;bar=1;baz=xyz
 ```
 
 ### `mimeParams.values()`