doc: correct spelling on placeholder
"placeholder" (no space) is already used in the spec and seems to be
the preferred option.
Removed space from "place holder".
Change-Id: I9b98f62f0e3f5adb019b99f5271cc9d19abf505e
GitHub-Last-Rev:
ed5aaf9d02c294e87688066f6218e5d58b0f62bf
GitHub-Pull-Request: golang/go#59626
Reviewed-on: https://go-review.googlesource.com/c/go/+/484576
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>