runtime(doc): Correct shell command to get $VIMRUNTIME into shell

runtime(doc): Correct shell command to get $VIMRUNTIME into shell

The original shell command could set the VIMRUNTIME variable to an
invalid path.
    - since v9.0.0592 trailing spaces were typically added making the
      path invalid. (I observed that spaces were added if the length of
      the path wasn't equal to one less than an integer multiple of the
      number of display columns.)
    - newline character/s would be added, making the path invalid, if
      the path was longer than the number of display columns.

The new command is shorter than the original command.

closes: #15184

Signed-off-by: Shane Harper <shane@shaneharper.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>