Fix a minor mistake in String::try_reserve_exact examples

Author: SpriteOvO

library/alloc/src/string.rs

@@ -1062,7 +1062,7 @@ impl String {
     ///     let mut output = String::new();
     ///
     ///     // Pre-reserve the memory, exiting if we can't
-    ///     output.try_reserve(data.len())?;
+    ///     output.try_reserve_exact(data.len())?;
     ///
     ///     // Now we know this can't OOM in the middle of our complex work
     ///     output.push_str(data);