installShellFiles.section.md: fix `unde[r]sirable` typo (#461892)

{

  nativeBuildInputs = [ installShellFiles ];

  # Sometimes the manpage file has an undersirable name; e.g., it conflicts with

  # Sometimes the manpage file has an undesirable name; e.g., it conflicts with

  # another software with an equal name. To install it with a different name,

  # the installed name must be provided before the path to the file.

  #