_content/doc: modernize spelling of "boilerplate"

Change-Id: Ic3346a42c91b0bb12904b382ef86b24c2db63b70
Reviewed-on: https://go-review.googlesource.com/c/website/+/477515
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Rob Pike <r@golang.org>

_content/doc/effective_go.html

@@ -1071,7 +1071,7 @@ func NewFile(fd int, name string) *File {
 </pre>
 
 <p>
-There's a lot of boiler plate in there.  We can simplify it
+There's a lot of boilerplate in there.  We can simplify it
 using a <i>composite literal</i>, which is
 an expression that creates a
 new instance each time it is evaluated.