Small grammar fix to docs for String::new().

Author: adeschamps

src/liballoc/string.rs

@@ -364,7 +364,7 @@ impl String {
     ///
     /// Given that the `String` is empty, this will not allocate any initial
     /// buffer. While that means that this initial operation is very
-    /// inexpensive, but may cause excessive allocation later, when you add
+    /// inexpensive, it may cause excessive allocation later when you add
     /// data. If you have an idea of how much data the `String` will hold,
     /// consider the [`with_capacity`] method to prevent excessive
     /// re-allocation.