docs/vm/hmm.rst: Add missed space

This space missing caused the colour scheme in vim editor messy
after that line. Add it to fix.

Signed-off-by: Baoquan He <bhe@redhat.com>
[ jc: fixed alignment after the changed line ]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/vm/hmm.rst

@@ -194,13 +194,13 @@ use either::
                       unsigned long start,
                       unsigned long end,
                       hmm_pfn_t *pfns);
- int hmm_vma_fault(struct vm_area_struct *vma,
-                   struct hmm_range *range,
-                   unsigned long start,
-                   unsigned long end,
-                   hmm_pfn_t *pfns,
-                   bool write,
-                   bool block);
+  int hmm_vma_fault(struct vm_area_struct *vma,
+                    struct hmm_range *range,
+                    unsigned long start,
+                    unsigned long end,
+                    hmm_pfn_t *pfns,
+                    bool write,
+                    bool block);
 
 The first one (hmm_vma_get_pfns()) will only fetch present CPU page table
 entries and will not trigger a page fault on missing or non-present entries.