README: Fix documentation to disable overscan

To disable overscan, DISABLE_OVERSCAN should be set to 1 rather than 0.

Signed-off-by: Jonathan Liu <net147@gmail.com>

README

@@ -105,7 +105,7 @@ KEY_DECODE_WVC1 = "0x12345678,0xabcdabcd,0x87654321"
 By default the GPU adds a black border around the video output to compensate for
 TVs which cut off part of the image. To disable this set this variable in
 local.conf:
-DISABLE_OVERSCAN = "0"
+DISABLE_OVERSCAN = "1"
 
 2.E. Optional - Set overclocking options:
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