Update Concepts/Linux-cpu-2.md cpumask

In x86_64 architecture `unsigned long` is 8-bytes size, so
 BITS_PER_BYTE * sizeof(long) = 64

Concepts/linux-cpu-2.md

@@ -59,7 +59,7 @@ where `BITS_TO_LONGS`:
 As we are focusing on the `x86_64` architecture, `unsigned long` is 8-bytes size and our array will contain only one element:
 
 ```
-(((8) + (8) - 1) / (8)) = 1
+(((8) + (64) - 1) / (64)) = 1
 ```
 
 `NR_CPUS` macro represents the number of CPUs in the system and depends on the `CONFIG_NR_CPUS` macro which is defined in [include/linux/threads.h](https://github.com/torvalds/linux/blob/16f73eb02d7e1765ccab3d2018e0bd98eb93d973/include/linux/threads.h) and looks like this:

contributors.md

@@ -132,3 +132,4 @@ Thank you to all contributors:
 * [Zhouyi Zhou](https://github.com/zhouzhouyi-hub)
 * [Mingzhe Yang](https://github.com/Mutated1994)
 * [Yuxin Wu](https://github.com/chaffz)
+* [Biao Ding](https://github.com/SmallPond)