Update instructions.append.md (#741)

* Update instructions.append.md

- Fix typo and clarify instructions in usage text
- Corrected a typo ("treat is" ➝ "treat it") in the program instructions.
- Added the word "code" at the end of the sentence "exit with a non-zero status" for clarity.

* Update instructions.append.md

- Fix typo and clarify instructions in usage text
- Corrected a typo ("treat is" ➝ "treat it") in the program instructions.

Author: OmkarBhosekar7711

exercises/practice/error-handling/.docs/instructions.append.md

@@ -12,5 +12,5 @@ This is different from passing no arguments at all.
 
 `./program`
 
-If your program is run with exactly one argument (even if it is an empty string), treat is as a person's name and print a greeting message.
+If your program is run with exactly one argument (even if it is an empty string), treat it as a person's name and print a greeting message.
 If it is run with zero arguments or more than one argument, print an error message and exit with a non-zero status.