Docs: Correct cmd_psuh and Explain UNUSED macro

The `cmd_psuh` function signature in the documentation,
was missing the `struct repository *repo` argument,
which is standard for built-in commands.
This commit corrects the signature to include the `repo` argument.

Additionally, this commit adds an explanation,
for using the `UNUSED` macro to prevent compiler warnings.

This helps new contributors understand,
common practices in the Git codebase.

Signed-off-by: K Jayatheerth <[email protected]>

Author: jayatheerthkulkarni

Documentation/MyFirstContribution.adoc

@@ -142,7 +142,15 @@ command in `builtin/psuh.c`. Create that file, and within it, write the entry
 point for your command in a function matching the style and signature:
 
 ----
-int cmd_psuh(int argc, const char **argv, const char *prefix)
+int cmd_psuh(int argc, const char **argv, const char *prefix, struct repository *repo)
+----
+
+We also use the UNUSED macro to make sure we don't recieve compiler warnings
+for unused arguments from the function cmd_psuh.
+
+----
+int cmd_psuh(int argc UNUSED, const char **argv UNUSED, 
+			const char *prefix UNUSED, struct repository *repo UNUSED)
 ----
 
 We'll also need to add the declaration of psuh; open up `builtin.h`, find the