Fix spelling (#3125)

include/nlohmann/detail/output/binary_writer.hpp

@@ -1535,7 +1535,7 @@ class binary_writer
     @tparam OutputIsLittleEndian Set to true if output data is
                                  required to be little endian
 
-    @note This function needs to respect the system's endianess, because bytes
+    @note This function needs to respect the system's endianness, because bytes
           in CBOR, MessagePack, and UBJSON are stored in network order (big
           endian) and therefore need reordering on little endian systems.
     */
@@ -1625,8 +1625,8 @@ class binary_writer
     }
 
   private:
-    /// whether we can assume little endianess
-    const bool is_little_endian = little_endianess();
+    /// whether we can assume little endianness
+    const bool is_little_endian = little_endianness();
 
     /// the output
     output_adapter_t<CharType> oa = nullptr;