docs: fix typos found by misspellings

docs: fix typos found by misspellings

The tool misspellings (https://github.com/lyda/misspell-check)
detected several typos. Command used:

  $ git ls-files | grep -v ^po/ | misspellings -f -

* Documentation/releases/v2.18-ReleaseNotes: Fix typo in news entry.
* NEWS: Likewise.
* config/texinfo.tex: Fix typo in comments.
* libmount/src/context.c: Fix typo in comment.
* libmount/src/fs.c: Likewise.
* login-utils/login.c: Likewise.
* login-utils.su.1: Fix typo in man page.
* sys-utils/chcpu.c: Fix typo in error message.

Signed-off-by: Bernhard Voelker <mail@bernhard-voelker.de>