[ICRC-1] Update metadata icrc1:logo for correcting value

[baolongt]

Hi!

Some dev don't know what value to put in the icrc1:logo it can be Data URL or URL to the image. We discussed in here

I propose a change in table Standard metadata entries

The URL of the token logo. The value can contain the actual image if it's a [Data URL](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URLs).

to something more clear what value should put here