Update without_std.rs (#2990)

fix typo

substrate/core/sr-std/without_std.rs

@@ -68,7 +68,7 @@ pub use core::slice;
 // Allow intepreting vectors of bytes as strings, but not constructing them.
 pub use core::str;
 // We are trying to avoid certain things here, such as `core::string`
-// (if you need `String` you most probably doing something wrong, since
+// (if you need `String` you are probably doing something wrong, since
 // runtime doesn't require anything human readable).
 
 pub mod collections {