Help: Remove outdated statement about get_filename_component()

After !2853, the statement that the "longest file extension is
always considered" is no longer true. Both NAME_WLE and
LAST_EXT now return details based on the shortest file
extension.

Fixes: 7a25ef326b (Help: Add documentation for new get_filename_component components, 2019-01-24, 3.14.0-rc1)

Help/command/get_filename_component.rst

@@ -15,14 +15,13 @@ Sets ``<var>`` to a component of ``<FileName>``, where ``<mode>`` is one of:
  NAME      = File name without directory
  EXT       = File name longest extension (.b.c from d/a.b.c)
  NAME_WE   = File name without directory or longest extension
- LAST_EXT  = File name last extention (.c from d/a.b.c)
+ LAST_EXT  = File name last extension (.c from d/a.b.c)
  NAME_WLE  = File name without directory or last extension
  PATH      = Legacy alias for DIRECTORY (use for CMake <= 2.8.11)
 
 Paths are returned with forward slashes and have no trailing slashes.
-The longest file extension is always considered.  If the optional
-``CACHE`` argument is specified, the result variable is added to the
-cache.
+If the optional ``CACHE`` argument is specified, the result variable is
+added to the cache.
 
 .. code-block:: cmake