config: Change "filesystem bundle" -> "filesystem"

The filesystem at root.path is not the bundle; the bundle contains
both root.path and the config.json as siblings (according to
bundle.md).  Remove the possibly confusing use of "bundle" for
something else, rolling back part of 718f9f3 (minor narrative cleanup
regarding config compatibility, 2017-01-30, opencontainers#673).

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -325,11 +325,11 @@ For Windows based systems the user structure has the following fields:
 
 **`platform`** (object, REQUIRED) specifies the configuration's target platform.
 
-* **`os`** (string, REQUIRED) specifies the operating system family of the container configuration's specified [`root`](#root) filesystem bundle.
+* **`os`** (string, REQUIRED) specifies the operating system family of the container configuration's specified [`root.path`](#root) filesystem.
     The runtime MUST generate an error if it does not support the specified **`os`**.
     Values SHOULD be, and runtimes SHOULD understand, **`os`** entries listed in the Go Language document for [`GOOS`][go-environment].
     If an operating system is not included in the `GOOS` documentation, it SHOULD be submitted to this specification for standardization.
-* **`arch`** (string, REQUIRED) specifies the instruction set for which the binaries in the specified [`root`](#root) filesystem bundle have been compiled.
+* **`arch`** (string, REQUIRED) specifies the instruction set for which the binaries in the specified [`root.path`](#root) filesystem have been compiled.
     The runtime MUST generate an error if it does not support the specified **`arch`**.
     Values SHOULD be, and runtimes SHOULD understand, **`arch`** entries listed in the Go Language document for [`GOARCH`][go-environment].
     If an architecture is not included in the `GOARCH` documentation, it SHOULD be submitted to this specification for standardization.