docs: remove non-breaking space from xen-command-line

Fixes rendering of the generated html.

Fixes: 31d9c88a38 ("pdx: introduce command line compression toggle")
Signed-of-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

docs/misc/xen-command-line.pandoc

@@ -2082,7 +2082,7 @@ for all of them (`true`), only for those subject to XPTI (`xpti`) or for
 those not subject to XPTI (`no-xpti`). The feature is used only in case
 INVPCID is supported and not disabled via `invpcid=false`.
 
-### pdx-compress
+### pdx-compress
 > `= <boolean>`
 
 > Default: `true` if CONFIG_PDX_NONE is unset