hwmon: (max6639) Update Datasheet URL

The old Datasheet does not exist anymore.
Signed-off-by: Marcello Sylvester Bauer <sylv@sylv.io>
Link: https://lore.kernel.org/r/76025f40d2684dc0d3ec02c8899b726b07a0e7da.1643299570.git.sylv@sylv.ioSigned-off-by: Guenter Roeck <linux@roeck-us.net>

Documentation/hwmon/max6639.rst

@@ -9,7 +9,7 @@ Supported chips:
 
     Addresses scanned: I2C 0x2c, 0x2e, 0x2f
 
-    Datasheet: http://pdfserv.maxim-ic.com/en/ds/MAX6639.pdf
+    Datasheet: https://datasheets.maximintegrated.com/en/ds/MAX6639-MAX6639F.pdf
 
 Authors:
     - He Changqing <hechangqing@semptian.com>