Fix data types indication

Fix the data types indication in basic examples of the Trait std::fmt::LowerExp and std::fmt::UpperExp.
Since there aren’t any type annotation on the let statement using the number 42.0, they are of type f64 according to The Book:
https://doc.rust-lang.org/book/ch03-02-data-types.html#floating-point-types

src/libcore/fmt/mod.rs

@@ -886,7 +886,7 @@ pub trait Pointer {
 ///
 /// # Examples
 ///
-/// Basic usage with `i32`:
+/// Basic usage with `f64`:
 ///
 /// ```
 /// let x = 42.0; // 42.0 is '4.2e1' in scientific notation
@@ -929,7 +929,7 @@ pub trait LowerExp {
 ///
 /// # Examples
 ///
-/// Basic usage with `f32`:
+/// Basic usage with `f64`:
 ///
 /// ```
 /// let x = 42.0; // 42.0 is '4.2E1' in scientific notation