[libc++][doc] Fixes grammar issues.

Author: mordante

libcxx/docs/ReleaseNotes/18.rst

@@ -144,7 +144,7 @@ ABI Affecting Changes
 ---------------------
 
 - The symbol of a non-visible function part of ``std::system_error`` was removed.
-  This is not a breaking change as the private function ``__init`` was never referenced internally outside of the dylib
+  This is not a breaking change as the private function ``__init`` was never referenced internally outside of the dylib.
 
 - This release of libc++ added missing visibility annotations on some types in the library. Users compiling with
   ``-fvisbility=hidden`` may notice that additional type infos from libc++ are being exported from their ABI. This is
@@ -159,7 +159,7 @@ ABI Affecting Changes
   extremely rare. Any error resulting from this change should result in a link-time error.
 
 - Under the unstable ABI, the internal alignment requirements for heap allocations
-  inside ``std::string`` has decreased from 16 to 8 This save memory since string requests fewer additional
+  inside ``std::string`` has decreased from 16 to 8. This saves memory since string requests fewer additional
   bytes than it did previously. However, this also changes the return value of ``std::string::max_size``
   and can cause code compiled against older libc++ versions but linked at runtime to a new version
   to throw a different exception when attempting allocations that are too large