fix wrong word in documentation
Change "two" to "three", since there are three significantly different things printed below that sentence:
---
While these:
```rust
println!("{}, `{name:.*}` has 3 fractional digits", "Hello", 3, name=1234.56);
println!("{}, `{name:.*}` has 3 characters", "Hello", 3, name="1234.56");
println!("{}, `{name:>8.*}` has 3 right-aligned characters", "Hello", 3, name="1234.56");
```
print two significantly different things:
``` rust
Hello, `1234.560` has 3 fractional digits
Hello, `123` has 3 characters
Hello, ` 123` has 3 right-aligned characters
```
---
[`https://doc.rust-lang.org/std/fmt/#precision`](https://doc.rust-lang.org/std/fmt/#precision)
2cc7da6f95