[libc++][NFC] Remove trailing whitespace from release notes

Author: ldionne

libcxx/docs/ReleaseNotes/18.rst

@@ -127,7 +127,7 @@ Improvements and New Features
 
 - AddressSanitizer annotations have been added to ``std::basic_string``.
 
-- The libc++ source code has been formatted with ``clang-format``. This 
+- The libc++ source code has been formatted with ``clang-format``. This
   `discourse thread <https://discourse.llvm.org/t/rfc-clang-formatting-all-of-libc-once-and-for-all>`_
   contains information how to rebase downstream patches.