Fails to compile

[nikivdev]

❯ cargo build --release
   Compiling proc-macro2 v0.4.26
   Compiling quote v0.6.11
   Compiling syn v0.15.26
   Compiling structopt-derive v0.2.14
   Compiling structopt v0.2.14
   Compiling mdck v0.1.1 (/Users/nikivi/clones/mdck)
warning: variable does not need to be mutable
  --> src/error.rs:15:13
   |
15 |         let mut walkdir_err;
   |             ----^^^^^^^^^^^
   |             |
   |             help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

    Finished release [optimized] target(s) in 12.00s