Migrate away from pivotal-legacy/homebrew-tap

Update reference documentation to the new spring-io/homebrew-tap repo.

Closes gh-24659

Author: philwebb

spring-boot-project/spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -291,8 +291,8 @@ If you are on a Mac and use https://brew.sh/[Homebrew], you can install the Spri
 
 [indent=0]
 ----
-	$ brew tap pivotal/tap
-	$ brew install springboot
+	$ brew tap spring-io/tap
+	$ brew install spring-boot
 ----
 
 Homebrew installs `spring` to `/usr/local/bin`.