library: Fix typo in the documentatio of Cstring::from_vec_with_nul

Fixes the sentence "Attempts to converts a Vec<u8> to a CString.", where "converts" should be in the base form as it is part of the to-infinitive form.

library/alloc/src/ffi/c_str.rs

@@ -636,7 +636,7 @@ impl CString {
         Self { inner: v.into_boxed_slice() }
     }
 
-    /// Attempts to converts a <code>[Vec]<[u8]></code> to a [`CString`].
+    /// Attempts to convert a <code>[Vec]<[u8]></code> to a [`CString`].
     ///
     /// Runtime checks are present to ensure there is only one nul byte in the
     /// [`Vec`], its last element.