Correct servertype identifier in "BlackMagic" programmer definition

When Arduino IDE initializes the integrated sketch debugger, it sets the `servertype` debug attribute to the value of the `debug.server` platform property.

This property must be set to one of the GDB Server type identifiers recognized by the Cortex-Debug debugger extension.

The previous value was not a recognized identifier, which caused the debugger initialization to fail:

Invalid servertype parameters. The following values are supported: "jlink", "openocd", "stlink", "stutil", "pyocd", "bmp", "pe", "qemu", "external"

Author: per1234

programmers.txt

@@ -35,4 +35,4 @@ blackmagic.program.protocol=
 blackmagic.program.tool=
 blackmagic.program.tool.default=
 blackmagic.program.extra_params=
-blackmagic.debug.server=blackmagic
+blackmagic.debug.server=bmp