syscall: fix typo in documentation for StringToUTF16Ptr

Fixes #20133

Change-Id: Ic1a6eb35de1f9ddac9527335eb49bf0b52963b6a
Reviewed-on: https://go-review.googlesource.com/41992Reviewed-by: Rob Pike <r@golang.org>

src/syscall/syscall_windows.go

@@ -57,7 +57,7 @@ func UTF16ToString(s []uint16) string {
 
 // StringToUTF16Ptr returns pointer to the UTF-16 encoding of
 // the UTF-8 string s, with a terminating NUL added. If s
-// If s contains a NUL byte this function panics instead of
+// contains a NUL byte this function panics instead of
 // returning an error.
 //
 // Deprecated: Use UTF16PtrFromString instead.