Fix typo in String::try_reserve_exact docs

Copying the pattern from `Vec::try_reserve_exact` and `String::try_reserve`,
it looks like this doc comment is intending to refer to the currently-being-documented
function.

Author: lopopolo

library/alloc/src/string.rs

@@ -1038,7 +1038,7 @@ impl String {
     }
 
     /// Tries to reserve the minimum capacity for exactly `additional` more elements to
-    /// be inserted in the given `String`. After calling `reserve_exact`,
+    /// be inserted in the given `String`. After calling `try_reserve_exact`,
     /// capacity will be greater than or equal to `self.len() + additional`.
     /// Does nothing if the capacity is already sufficient.
     ///