binary cache docs: remove redundant flag and comment (#38960)

lib/spack/docs/binary_caches.rst

@@ -48,15 +48,11 @@ Here is an example where a build cache is created in a local directory named
 
 .. code-block:: console
 
-    $ spack buildcache push --allow-root ./spack-cache ninja
+    $ spack buildcache push ./spack-cache ninja
     ==> Pushing binary packages to file:///home/spackuser/spack/spack-cache/build_cache
 
 Not that ``ninja`` must be installed locally for this to work.
 
-We're using the ``--allow-root`` flag to tell Spack that is OK when any of
-the binaries we're pushing contain references to the local Spack install
-directory.
-
 Once you have a build cache, you can add it as a mirror, discussed next.
 
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