doc: fix small typo in doc

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/170660043

doc/install.html

@@ -47,7 +47,7 @@ <h2 id="requirements">System requirements</h2>
 <tr><td>FreeBSD 8 or later</td> <td>amd64, 386, arm</td> <td>Debian GNU/kFreeBSD not supported; FreeBSD/ARM needs FreeBSD 10 or later</td></tr>
 <tr><td>Linux 2.6.23 or later with glibc</td> <td>amd64, 386, arm</td> <td>CentOS/RHEL 5.x not supported; no binary distribution for ARM yet</td></tr>
 <tr><td>Mac OS X 10.6 or later</td> <td>amd64, 386</td> <td>use the gcc<sup>&#8224;</sup> that comes with Xcode<sup>&#8225;</sup></td></tr>
-<tr><td>Windows XP or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cgywin or msys.</td></tr>
+<tr><td>Windows XP or later</td> <td>amd64, 386</td> <td>use MinGW gcc<sup>&#8224;</sup>. No need for cygwin or msys.</td></tr>
 </table>
 
 <p>