Fix format in constants.md

Author: YXHXianYu

src/custom_types/constants.md

@@ -4,7 +4,7 @@ Rust has two different types of constants which can be declared in any scope
 including global. Both require explicit type annotation:
 
 * `const`: An unchangeable value (the common case).
-* `static`: A possibly `mut`able variable with [`'static`][static] lifetime.
+* `static`: A possibly mutable variable with [`'static`][static] lifetime.
   The static lifetime is inferred and does not have to be specified.
   Accessing or modifying a mutable static variable is [`unsafe`][unsafe].