patch 9.1.1319: Various typos in the code, issue with test_inst_complete.vim

Problem:  Various typos in the code, redundant and strange use of
          :execute in test_ins_complete.vim (after 9.1.1315).
Solution: Fix typos in the code and in the documentation, use the
          executed command directly (zeertzjq).

closes: #17143

Co-authored-by: Christ van Willegen <cvwillegen@gmail.com>
Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/term.c

@@ -1710,7 +1710,7 @@ static char *(key_names[]) =
     // Do those ones first, both may cause a screen redraw.
     "Co",
     // disabled, because it switches termguicolors, but that
-    // is noticable and confuses users
+    // is noticeable and confuses users
     // "RGB",
 # endif
     "ku", "kd", "kr", "kl",