std::fmt: Use args directly in example code

The lint "clippy::uninlined_format_args" recommends inline
variables in format strings. Fix two places in the docs that do
not do this. I noticed this because I copy/pasted one example in
to my project, then noticed this lint error. This fixes:

error: variables can be used directly in the `format!` string
  --> src/main.rs:30:22
   |
30 |         let string = format!("{:.*}", decimals, magnitude);
   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: variables can be used directly in the `format!` string
  --> src/main.rs:39:2
   |
39 |  write!(&mut io::stdout(), "{}", args).unwrap();

0 files changed, 0 insertions, 0 deletions