unix: fix minor typos in README.md

Change-Id: Ibf1ce4330e196578c933784b51fff7f6670e2fa5
Reviewed-on: https://go-review.googlesource.com/c/sys/+/324449
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/README.md

@@ -76,7 +76,7 @@ arguments can be passed to the kernel. The third is for low-level use by the
 ForkExec wrapper. Unlike the first two, it does not call into the scheduler to
 let it know that a system call is running.
 
-When porting Go to an new architecture/OS, this file must be implemented for
+When porting Go to a new architecture/OS, this file must be implemented for
 each GOOS/GOARCH pair.
 
 ### mksysnum
@@ -107,7 +107,7 @@ prototype can be exported (capitalized) or not.
 Adding a new syscall often just requires adding a new `//sys` function prototype
 with the desired arguments and a capitalized name so it is exported. However, if
 you want the interface to the syscall to be different, often one will make an
-unexported `//sys` prototype, an then write a custom wrapper in
+unexported `//sys` prototype, and then write a custom wrapper in
 `syscall_${GOOS}.go`.
 
 ### types files
@@ -137,7 +137,7 @@ some `#if/#elif` macros in your include statements.
 
 This script is used to generate the system's various constants. This doesn't
 just include the error numbers and error strings, but also the signal numbers
-an a wide variety of miscellaneous constants. The constants come from the list
+and a wide variety of miscellaneous constants. The constants come from the list
 of include files in the `includes_${uname}` variable. A regex then picks out
 the desired `#define` statements, and generates the corresponding Go constants.
 The error numbers and strings are generated from `#include <errno.h>`, and the