README: Replace `git://` with `https://`

- git:// has been deprecated by GitHub:
   *  fatal: remote error:
      The unauthenticated git protocol on port 9418 is no longer supported.
      Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.
      manifests: sleeping 4.0 seconds before retrying

- Switch to https://, since people may get confused while copy-pasting commands.

README.md

@@ -53,11 +53,11 @@ with [Git and Repo](https://source.android.com/source/using-repo.html).
 
 To initialize your local repository using the AOSP trees to build TWRP, use a command like this:
 
-    repo init -u git://github.com/minimal-manifest-twrp/platform_manifest_twrp_aosp.git -b twrp-11
+    repo init -u https://github.com/minimal-manifest-twrp/platform_manifest_twrp_aosp.git -b twrp-11
 
 To initialize a shallow clone, which will save even more space, use a command like this:
 
-    repo init --depth=1 -u git://github.com/minimal-manifest-twrp/platform_manifest_twrp_aosp.git -b twrp-11
+    repo init --depth=1 -u https://github.com/minimal-manifest-twrp/platform_manifest_twrp_aosp.git -b twrp-11
 
 Then to sync up: