Fix typo in usage text

Author: dimbleby

src/bin/main.rs

@@ -87,7 +87,7 @@ fn make_opts() -> Options {
     opts.optflag(
         "",
         "check",
-        "Run in 'check' mode. Exits with 0 if input if formatted correctly. Exits \
+        "Run in 'check' mode. Exits with 0 if input is formatted correctly. Exits \
          with 1 and prints a diff if formatting is required.",
     );
     let is_nightly = is_nightly();