Change Spresense firmware download link

Author: kamtom480

ports/cxd56/README.md

@@ -75,7 +75,7 @@ Bootloader information:
 
 * You have to accept the End User License Agreement to be able to download and use the Spresense bootloader binary.
 
-Download the spresense binaries zip archive from: <https://developer.sony.com/file/download/download-spresense-firmware-v1-4-000>
+Download the spresense binaries zip archive from: [Spresense firmware v1-4-000](https://developer.sony.com/file/download/download-spresense-firmware-v1-4-000)
 
 Extract spresense binaries in your PC to ports/spresense/spresense-exported-sdk/firmware/