Update tiktok.svg (#532)

* Update tiktok.svg

This file is bigger by 38 bytes (446 => 484) but it avoids the use of `<!ENTITY ...>` and `<!DOCTYPE ...>` and as consequence:
- It respect the template starting with `<svg xmlns="http://www.w3.org/2000/svg" ...>`.
- This `svg` code can be integrated directly in `html`.

* Update README.md

Update the size of `tiktok.svg`.

images/svg/tiktok.svg

@@ -1,7 +1,5 @@
-<!DOCTYPE a [<!ENTITY z "m 219,200 a 117,117 0 1 0 100.6,115.4 v -128 a 150,150 0 0 0 88,28 v -63 a 88,88 0 0 1 -88,-88 H 256 V 316 a 53.5,53.5 0 1 1 -37,-51 z">]>
-<svg xmlns="http://www.w3.org/2000/svg"
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"
 aria-label="TikTok" role="img"
 viewBox="0 0 512 512"><rect
-rx="15%"
-height="512"
-width="512" fill="#fff"/><style>*{mix-blend-mode:multiply}</style><path fill="#ff004f" d="&z;" transform="translate(18,15)"/><path d="&z;" fill="#00f2ea"/></svg>
+rx="15%" height="512" width="512"
+fill="#fff"/><defs><path id="t" d="m 219,200 a 117,117 0 1 0 100.6,115.4 v -128 a 150,150 0 0 0 88,28 v -63 a 88,88 0 0 1 -88,-88 H 256 V 316 a 53.5,53.5 0 1 1 -37,-51 z" style="mix-blend-mode:multiply"/></defs><use href="#t" fill="#ff004f" transform="translate(18,15)"/><use href="#t" fill="#00f2ea"/></svg>