Update let.md -- follow whitespace style guideline

"Idiomatic code should not use extra whitespace in the middle of a line to provide alignment."

src/doc/style/features/let.md

@@ -34,7 +34,7 @@ Prefer
 
 ```rust
 let foo = match bar {
-    Baz  => 0,
+    Baz => 0,
     Quux => 1
 };
 ```