Correct description of SC clock speed switch (#548)

Author: quinnyo

src/Serial_Data_Transfer_(Link_Cable).md

@@ -37,7 +37,7 @@ incoming bit is shifted in from the other side:
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 - **Transfer enable** (*Read/Write*): If `1`, a transfer is either requested or in progress.
-- **Clock speed** \[*CGB Mode only*\] (*Read/Write*): If set to `1`, the internal clock's speed is doubled.
+- **Clock speed** \[*CGB Mode only*\] (*Read/Write*): If set to `1`, enable high speed serial clock (~256 kHz in single-speed mode)
 - **Clock select** (*Read/Write*): `0` = External clock ("slave"), `1` = Internal clock ("master").
 
 The master Game Boy will load up a data byte in SB and then set