docs/cli: Align and fix cli documentation

This changes the deprecated :show-nested: instruction to the :nested: one [1].
This also fixes warning about misindented block [2].

[1] https://sphinx-click.readthedocs.io/en/latest/usage/#directive-click

[2]
```
06:22:34  swh fs mount:1:Unexpected indentation.
06:22:34  make: *** [../../swh-docs/Makefile.sphinx:32: sphinx/html] Error 2
```

docs/cli.rst

@@ -5,4 +5,4 @@ Command-line interface
 
 .. click:: swh.fuse.cli:fuse
   :prog: swh fs
-  :show-nested:
+  :nested: full

swh/fuse/cli.py

@@ -106,8 +106,10 @@ def fuse(ctx, config_file):
     "-f/-d",
     "--foreground/--daemon",
     default=False,
-    help="whether to run FUSE attached to the console (foreground) "
-    "or daemonized in the background (default: daemon)",
+    help=(
+        "whether to run FUSE attached to the console (foreground) "
+        "or daemonized in the background (default: daemon)"
+    ),
 )
 @click.pass_context
 def mount(ctx, swhids, path, foreground):
@@ -117,10 +119,10 @@ def mount(ctx, swhids, path, foreground):
     populate the virtual file system (VFS). Otherwise the VFS will be populated
     on-demand, when accessing its content.
 
-    \b
     Example:
 
-    \b
+    .. code-block:: bash
+
       $ mkdir swhfs
       $ swh fs mount swhfs/
       $ grep printf swhfs/archive/swh:1:cnt:c839dea9e8e6f0528b468214348fee8669b305b2