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/os_unix.c

@@ -4434,7 +4434,7 @@ mch_report_winsize(int fd, int rows, int cols)
     ws.ws_col = cols;
     ws.ws_row = rows;
 
-    // calcurate and set tty pixel size
+    // calculate and set tty pixel size
     struct cellsize cs;
     mch_calc_cell_size(&cs);