Documentation/memory_mapping: Insert missing dash

The description of 'remap_pfn_range()' parameters inserts dash between
the name of each parameter and the description but it is missed for
'pfn' parameter.  This commit adds the missed dash for 'pfn'.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>

Author: sjp38

Documentation/teaching/labs/memory_mapping.rst

@@ -208,7 +208,7 @@ into the virtual space represented by :c:type:`vm_area_struct`:
 * *addr* - the virtual address space from where remapping begins; page
   tables for the virtual address space between addr and addr + size
   will be formed as needed
-* *pfn* the page frame number to which the virtual address should be
+* *pfn* - the page frame number to which the virtual address should be
   mapped
 * *size* - the size (in bytes) of the memory to be mapped
 * *prot* - protection flags for this mapping