Update docs link for `up space init` to point to correct URL.

[tnthornton]

Description of your changes

Pretty much what the summary says, the correct URL is https://docs.upbound.io/concepts/upbound-spaces.

I have:

• [X] Read and followed Upbound's contribution process.
• [X] Run make reviewable to ensure this PR is ready for review.

How has this code been tested

./_output/bin/darwin_arm64/up space init --token-file=key.json v0.14.0-23.ga50d1a87
 INFO  Required prerequisites met!
 INFO  Proceeding with Upbound Spaces installation...
  √   [1/3]: Creating pull secret upbound-pull-secret
  √   [2/3]: Initializing Space components
  √   [3/3]: Starting Space Components
  🙌  Your Upbound Space is Ready!

  👀  Next Steps 👇

👉 Check out Upbound Spaces docs @ https://docs.upbound.io/concepts/upbound-spaces