fix(docs): revert starter url in tutorial (#26528)

Code on L.214 doesn't work on my environment and this behaviour is expected to be common in many other environments.

Indeed, the page https://github.com/gatsbyjs/gatsby/tree/master/starters shows a proper code:
`gatsby new my-blazing-fast-site https://github.com/gatsbyjs/gatsby-starter-hello-world`,
instead of the previous one `gatsby new hello-world https://github.com/gatsbyjs/gatsby/tree/master/starters/hello-world`.

Then I propose the change also on the index.md.

Author: gatsbybot

yarn.lock

@@ -1644,12 +1644,7 @@
     "@types/node" "*"
     form-data "^3.0.0"
 
-"@types/node@*":
-  version "14.0.27"
-  resolved "https://registry.yarnpkg.com/@types/node/-/node-14.0.27.tgz#a151873af5a5e851b51b3b065c9e63390a9e0eb1"
-  integrity sha512-kVrqXhbclHNHGu9ztnAwSncIgJv/FaxmzXJvGXNdcCpV1b8u1/Mi6z6m0vwy0LzKeXFTPLH0NzwmoJ3fNCIq0g==
-
-"@types/node@^8.5.7":
+"@types/node@*", "@types/node@^8.5.7":
   version "8.10.62"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-8.10.62.tgz#8d0d0db44a46ae7679d92e2e36d41bf1c3625d6a"
   integrity sha512-76fupxOYVxk36kb7O/6KtrAPZ9jnSK3+qisAX4tQMEuGNdlvl7ycwatlHqjoE6jHfVtXFM3pCrCixZOidc5cuw==