Fix "try in playground" URLs broken when b64 contains +

[hchargois]

The "Try it in the playground" links in code blocks are broken when the base64 encoding of the code contains the character +, as it is a reserved character in URLs which gets replaced by a space on the playground's end, which completely breaks the code.

Example: the first code block of this section https://docs.vlang.io/functions-2.html#lambda-expressions

Should be

mut a := [1, 2, 3]
a.sort(|x, y| x > y) // sorts the array, defining the comparator with a lambda expression
println(a.map(|x| x * 10)) // prints [30, 20, 10]

But in the playground it opens like so:

Some blocks are even more broken, such as this one https://docs.vlang.io/concurrency.html#channel-select which makes the playground fail to display anything.

The fix is simple, correctly encode the query param value.