Fix up one mistake

`/proc/pid/systemcall` -> `/proc/${pid}/syscall`

SysCall/linux-syscall-1.md

@@ -182,7 +182,7 @@ The `ltrace` util displays a set of userspace calls of a program. The `fopen` fu
 write@SYS(1, "Hello World!\n\n", 14) = 14
 ```
 
-Yes, system calls are ubiquitous. Each program needs to open/write/read files and network connections, allocate memory, and many other things that can be provided only by the kernel. The [proc](https://en.wikipedia.org/wiki/Procfs) file system contains special files in a format: `/proc/pid/systemcall` that exposes the system call number and argument registers for the system call currently being executed by the process. For example, pid 1 is [systemd](https://en.wikipedia.org/wiki/Systemd) for me:
+Yes, system calls are ubiquitous. Each program needs to open/write/read files and network connections, allocate memory, and many other things that can be provided only by the kernel. The [proc](https://en.wikipedia.org/wiki/Procfs) file system contains special files in a format: `/proc/${pid}/syscall` that exposes the system call number and argument registers for the system call currently being executed by the process. For example, pid 1 is [systemd](https://en.wikipedia.org/wiki/Systemd) for me:
 
 ```
 $ sudo cat /proc/1/comm