Fix Documentation (InitialConfiguration) go backend (Cherry-pick of #19806)

[WorkerPants]

What

Update the initial-configuration documentation on https://www.pantsbuild.org/docs/initial-configuration

from pants.backend.go -> pants.backend.experimental.go

Why

According to https://www.pantsbuild.org/docs/enabling-backends pants.backend.experimental.go is the correct backend name. Also if you use pants.backend.go on 2.17.0 it fails building

$ cat pants.toml
[GLOBAL]
pants_version = "2.17.0"
backend_packages = [
  "pants.backend.go",
  "pants.backend.python",
  "pants.backend.python.lint.black",
]

$ pants tailor ::
13:03:35.59 [INFO] waiting for pantsd to start...
13:03:39.18 [INFO] pantsd started
Traceback (most recent call last):
  File "/Users/nobu/Library/Caches/nce/50b60c9f29c54578592fd2b4725722435f65d43cb21e6e2732268b6d5050a09a/bindings/venvs/2.17.0/lib/python3.9/site-packages/pants/init/extension_loader.py", line 141, in load_backend
    module = importlib.import_module(backend_module)
  File "/Users/nobu/Library/Caches/nce/3abc4d5fbbc80f5f848f280927ac5d13de8dc03aabb6ae65d8247cbb68e6f6bf/cpython-3.9.16+20230507-x86_64-apple-darwin-install_only.tar.gz/python/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 984, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'pants.backend.go.register'
13:03:39.23 [ERROR] Failed to load the pants.backend.go.register backend: ModuleNotFoundError("No module named 'pants.backend.go.register'")

When, I update this to the experimental version, it runs tailor correctly

 $ cat pants.toml
[GLOBAL]
pants_version = "2.17.0"
backend_packages = [
  "pants.backend.experimental.go",
  "pants.backend.python",
  "pants.backend.python.lint.black",
]

$ pants tailor ::
13:15:07.47 [INFO] waiting for pantsd to start...
13:15:10.84 [INFO] pantsd started
13:15:11.82 [INFO] Initializing scheduler...
13:15:20.25 [INFO] Scheduler initialized.