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>
diff --git a/src/diff.c b/src/diff.c
index e694cf2..ec108e9 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -3229,7 +3229,7 @@
 }
 
 /*
- * Find the inline difference within a diff block among differnt buffers.  Do
+ * Find the inline difference within a diff block among different buffers.  Do
  * this by splitting each block's content into characters or words, and then
  * use internal xdiff to calculate the per-character/word diff.  The result is
  * stored in dp instead of returned by the function.
@@ -4548,7 +4548,7 @@
     {
 	// Remember the results if using simple since it's recalculated per
 	// call. Otherwise just call diff_find_change() every time since
-	// internally the result is cached interally.
+	// internally the result is cached internally.
 	cache_results = FALSE;
     }