doc: fix two spelling mistakes in contribute.html

Change-Id: Ife60468d508f90321733ebdbdb0e6cf443ba1659
Reviewed-on: https://go-review.googlesource.com/82156Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/contribute.html

@@ -187,7 +187,7 @@ prints help text, not an error.
 On Windows, when using git-bash you must make sure that
 <code>git-codereview.exe</code> is in your git exec-path.
 Run <code>git --exec-path</code> to discover the right location then create a
-symbolic link or simply copy the executible from $GOPATH/bin to this directory.
+symbolic link or simply copy the executable from $GOPATH/bin to this directory.
 </p>
 
 <p>
@@ -386,7 +386,7 @@ Instead, your name will appear in the
 <a href="https://golang.org/change">change log</a> and in the <a
 href="/CONTRIBUTORS"><code>CONTRIBUTORS</code></a> file and perhaps the <a
 href="/AUTHORS"><code>AUTHORS</code></a> file.
-These files are automatically generated from the commit logs perodically.
+These files are automatically generated from the commit logs periodically.
 The <a href="/AUTHORS"><code>AUTHORS</code></a> file defines who &ldquo;The Go
 Authors&rdquo;&mdash;the copyright holders&mdash;are.
 </p>