Update README.md (#2258)

Replace `PWD` with correct `pwd`. 

Incorrect:

```sh
$ docker run --rm -v /var/run/docker.sock:/var/run/docker.sock:rw -v $(PWD):/tmp/lint:rw oxsecurity/megalinter:latest
PWD: command not found
Skipped setting git safe.directory DEFAULT_WORKSPACE:  ...
Setting git safe.directory default: /github/workspace ...
Setting git safe.directory to /tmp/lint ...
[MegaLinter init] ONE-SHOT RUN
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/runpy.py", line 196, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/megalinter/run.py", line 9, in <module>
    linter = megalinter.Megalinter({"cli": True})
  File "/megalinter/MegaLinter.py", line 56, in __init__
    self.workspace = self.get_workspace()
  File "/megalinter/MegaLinter.py", line 361, in get_workspace
    raise FileNotFoundError(
FileNotFoundError: [Context] Unable to find a workspace to lint 
DEFAULT_WORKSPACE: 
GITHUB_WORKSPACE:
```

Corrected
```sh
$ docker run --rm -v /var/run/docker.sock:/var/run/docker.sock:rw -v $(pwd):/tmp/lint:rw oxsecurity/megalinter:latest
Skipped setting git safe.directory DEFAULT_WORKSPACE:  ...
Setting git safe.directory default: /github/workspace ...
Setting git safe.directory to /tmp/lint ...
[MegaLinter init] ONE-SHOT RUN
[config] Environment variables only (no config file found in /tmp/lint)
----------------------------------------------------------------------------------------------------
------------------------------------ MegaLinter, by OX Security ------------------------------------
----------------------------------------------------------------------------------------------------
 - Image Creation Date: 2023-01-07T10:05:11Z
 - Image Revision: 8fd433c
 - Image Version: v6
----------------------------------------------------------------------------------------------------
The MegaLinter documentation can be found at:
 - https://megalinter.io/latest
...
```

README.md

@@ -729,14 +729,14 @@ The workflow above should only trigger on push, not on any other situation. For
 
 You can also run megalinter with its Docker container, just execute this command:
 
-`docker run --rm -v /var/run/docker.sock:/var/run/docker.sock:rw -v $(PWD):/tmp/lint:rw oxsecurity/megalinter:v6`
+`docker run --rm -v /var/run/docker.sock:/var/run/docker.sock:rw -v $(pwd):/tmp/lint:rw oxsecurity/megalinter:v6`
 
 **No extra arguments are needed,** however, megalinter will lint all of the files inside the `/tmp/lint` folder, so it may be needed to configure your tool of choice to use the `/tmp/lint` folder as workspace.
 This can also be changed:
 
 _Example:_
 
-`docker run --rm -v /var/run/docker.sock:/var/run/docker.sock:rw -v $(PWD):/example/folder:rw oxsecurity/megalinter:v6`
+`docker run --rm -v /var/run/docker.sock:/var/run/docker.sock:rw -v $(pwd):/example/folder:rw oxsecurity/megalinter:v6`
 
 ### Run MegaLinter locally