Fixup a typo in a comment in a test

Author: WaffleLapkin

tests/fail/tail_calls/signature-mismatch-arg.rs

@@ -3,7 +3,7 @@
 
 fn main() {
     // FIXME(explicit_tail_calls):
-    //   the error should point to `become f(x)`,
+    //   the error should point to `become g(x)`,
     //   but tail calls mess up the backtrace it seems like...
     f(0);
     //~^ error: Undefined Behavior: calling a function with argument of type i32 passing data of type u32