fix SPI documentation configuration

[mpospese]

Intro

Our repo contains a config file that tells the Swift Package Index where to point for our documentation, but it has a typo in it and so does not work.

Task

Fix the typo in .spi.yml config file

current text:

version: 1
external_links:
  documentation: "<https://yml-org.github.io/YCoreUI/">

new text:

version: 1
external_links:
  documentation: "https://yml-org.github.io/YCoreUI/"