doc/go1.17: fix typo "MacOS" -> "macOS"

Change-Id: Ie2ada2bf875a93b1cc9e86a81c8a25de39ce4752
GitHub-Last-Rev: 462753d
GitHub-Pull-Request: #46962
Reviewed-on: https://go-review.googlesource.com/c/go/+/331589
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: lescoggi

doc/go1.17.html

@@ -401,7 +401,7 @@ <h2 id="compiler">Compiler</h2>
 
 <p><!-- golang.org/issue/40724 -->
   Go 1.17 implements a new way of passing function arguments and results using
-  registers instead of the stack. This work is enabled for Linux, MacOS, and
+  registers instead of the stack. This work is enabled for Linux, macOS, and
   Windows on the 64-bit x86 architecture (the <code>linux/amd64</code>,
   <code>darwin/amd64</code>, <code>windows/amd64</code> ports). For a
   representative set of Go packages and programs, benchmarking has shown