functions: Fix command name

This was "function", needs to be "function*s*".

It was only an issue in the option parsing because we set cmd there
again instead of passing it. Maybe these should just be file-level constants?

fish-rust/src/builtins/functions.rs

@@ -71,7 +71,7 @@ fn parse_cmd_opts<'args>(
     parser: &mut parser_t,
     streams: &mut io_streams_t,
 ) -> Option<c_int> {
-    let cmd = L!("function");
+    let cmd = L!("functions");
     let print_hints = false;
     let mut w = wgetopter_t::new(SHORT_OPTIONS, LONG_OPTIONS, argv);
     while let Some(opt) = w.wgetopt_long() {

tests/checks/functions.fish

@@ -224,3 +224,8 @@ functions --details --verbose thisfunctiondoesnotexist
 # CHECK: 0
 # CHECK: n/a
 # CHECK: n/a
+
+functions --banana
+# CHECKERR: functions: --banana: unknown option
+echo $status
+# CHECK: 2