SpringBoot Fix dead link url and name of the STS download link

In the CONTRIBUTING documentation the link to the Spring Tool Suite url is a dead link. The url is replaced by the new url where STS can be downloaded. The name of this link is contains a typo: Tools Suite (with an s after the Tool) instead of Tool Suite.

Comment From: wilkinsona

@fernandezseb Thank you very much for making your first contribution to Spring Boot.