all: single space after period

Follow CL 20022 and consistently use single space after a period in
documentation.

Generated with:

$ perl -i -npe 's,^(\s*// .+[a-z]\.)  +([A-Z]),$1 $2,' $(git grep -l -E '^\s*//(.+\.)  +([A-Z])')

Change-Id: Ia29ad823668f060e81293e848a79fc4b4857d94b
Reviewed-on: https://go-review.googlesource.com/73530
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux_386.go

@@ -176,9 +176,9 @@ func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
 
 // On x86 Linux, all the socket calls go through an extra indirection,
 // I think because the 5-register system call interface can't handle
-// the 6-argument calls like sendto and recvfrom.  Instead the
+// the 6-argument calls like sendto and recvfrom. Instead the
 // arguments to the underlying system call are the number below
-// and a pointer to an array of uintptr.  We hide the pointer in the
+// and a pointer to an array of uintptr. We hide the pointer in the
 // socketcall assembly to avoid allocation on every system call.
 
 const (