Fix spelling

src/Compilers/Core/Portable/Serialization/ObjectReader.cs

@@ -38,7 +38,7 @@ internal sealed partial class ObjectReader : IDisposable
         /// <summary>
         /// Map of reference id's to deserialized objects.
         ///
-        /// These are not readonly because they're structs and we mutate htemthem.
+        /// These are not readonly because they're structs and we mutate them.
         /// </summary>
         private ReaderReferenceMap<object> _objectReferenceMap;
         private ReaderReferenceMap<string> _stringReferenceMap;

src/Compilers/Core/Portable/Serialization/ObjectWriter.cs

@@ -33,7 +33,7 @@ internal sealed partial class ObjectWriter : IDisposable
         /// for performance.  While the string-reference-map uses value-equality for greater cache hits 
         /// and reuse.
         /// 
-        /// These are not readonly because they're structs and we mutate htemthem.
+        /// These are not readonly because they're structs and we mutate them.
         /// </summary>
         private WriterReferenceMap _objectReferenceMap;
         private WriterReferenceMap _stringReferenceMap;