engine/security/rootless: fix reviewdog errors

```
Warning: [vale] reported by reviewdog 🐶
[Docker.Avoid] Consider removing 'very'.

Raw Output:
{"message": "[Docker.Avoid] Consider removing 'very'.", "location": {"path": "content/manuals/engine/security/rootless/_index.md", "range": {"start": {"line": 18, "column": 9}}}, "severity": "WARNING"}
reviewdog: found at least one issue with severity greater than or equal to the given level: warning
Error: [vale] reported by reviewdog 🐶
[Vale.Terms] Use 'cgroup' instead of 'Cgroup'.

Raw Output:
{"message": "[Vale.Terms] Use 'cgroup' instead of 'Cgroup'.", "location": {"path": "content/manuals/engine/security/rootless/troubleshoot.md", "range": {"start": {"line": 73, "column": 3}}}, "severity": "ERROR"}
```

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Author: AkihiroSuda

content/manuals/engine/security/rootless/_index.md

@@ -15,7 +15,7 @@ the Docker daemon, as long as the [prerequisites](#prerequisites) are met.
 ## How it works
 
 Rootless mode executes the Docker daemon and containers inside a user namespace.
-This is very similar to [`userns-remap` mode](../userns-remap.md), except that
+This is similar to [`userns-remap` mode](../userns-remap.md), except that
 with `userns-remap` mode, the daemon itself is running with root privileges,
 whereas in rootless mode, both the daemon and the container are running without
 root privileges.

content/manuals/engine/security/rootless/troubleshoot.md

@@ -70,7 +70,7 @@ weight: 30
   - `fuse-overlayfs` (only if running with kernel 4.18 or later, and `fuse-overlayfs` is installed)
   - `btrfs` (only if running with kernel 4.18 or later, or `~/.local/share/docker` is mounted with `user_subvol_rm_allowed` mount option)
   - `vfs`
-- Cgroup is supported only when running with cgroup v2 and systemd. See [Limiting resources](./tips.md#limiting-resources).
+- cgroup is supported only when running with cgroup v2 and systemd. See [Limiting resources](./tips.md#limiting-resources).
 - Following features are not supported:
   - AppArmor
   - Checkpoint