libgomp: Fix up some typos in libgomp.texi

I decided to check for repeated the the in libgomp and noticed
there are several occurrences of a typo theads rather than threads
in libgomp.texi.

2023-02-16  Jakub Jelinek  <jakub@redhat.com>

	* libgomp.texi: Fix typos - theads -> threads.