Resolve let_underscore_untyped pedantic clippy lint in test

    error: non-binding `let` without a type annotation
      --> tests/test_writedoc.rs:50:5
       |
    50 | /     let _ = writedoc! {
    51 | |         if ZERO < Pair::<fn() -> (), ()>::ONE { &mut s } else { &mut s },
    52 | |         "writedoc",
    53 | |     };
       | |______^
       |
       = help: consider adding a type annotation or removing the `let` keyword
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_underscore_untyped
       = note: `-D clippy::let-underscore-untyped` implied by `-D clippy::pedantic`

tests/test_writedoc.rs

@@ -47,7 +47,7 @@ fn test_angle_bracket_parsing() {
     }
 
     let mut s = String::new();
-    let _ = writedoc! {
+    _ = writedoc! {
         if ZERO < Pair::<fn() -> (), ()>::ONE { &mut s } else { &mut s },
         "writedoc",
     };