Remove duplicated char

Author: chansuke

compiler/rustc_error_codes/src/error_codes/E0778.md

@@ -5,7 +5,7 @@ Erroneous code example:
 ```compile_fail,E0778
 #![feature(isa_attribute)]
 
-#[instruction_set()] // error: `[instruction_set]` requires an argumentt
+#[instruction_set()] // error: `[instruction_set]` requires an argument
 pub fn something() {}
 fn main() {}
 ```