Fix in-band lifetimes example with backreferences

Backreferences are listed as a "possible extension or alternative" in
RFC rust-lang#2115, so the examples should not include them. This is further
reinforced by commit c20ea6d, which
appears to have been intended to remove all instances of
backreferences in the examples, but missed these. [@dhardy also
observed this issue]
(rust-lang#2115 (comment)).

text/2115-argument-lifetimes.md

@@ -334,8 +334,8 @@ tomorrow you would write:
 
 ```rust
 fn elided(&self) -> &str
-fn two_args(arg1: &Foo, arg2: &Bar) -> &'arg2 Baz
-fn two_lifetimes(arg1: &Foo, arg2: &Bar) -> &'arg1 Quux<'arg2>
+fn two_args(arg1: &Foo, arg2: &'a Bar) -> &'a Baz
+fn two_lifetimes(arg1: &'a Foo, arg2: &'b Bar) -> &'a Quux<'b>
 
 impl MyStruct<'A> {
     fn foo(&self) -> &'A str