Mended broken links in README.html

Mended some broken relative links in README.html that
pointed to other parts of the docs.

Also, rename "Mac OS X" to "macOS" in more places.

docs/en/html/OPTIONS.html

@@ -115,7 +115,7 @@
       </blockquote>
 
       <h2>
-        Mac&nbsp;OS&nbsp;X Users
+        macOS Users
       </h2>
 
       <blockquote>
@@ -389,7 +389,7 @@
             the <b>[Alt]</b> modifier key is being held. (This is
             typically used for providing a print dialog, similar to
             when pressing <b>[Alt]</b>+'Print' in Windows and
-            Mac&nbsp;OS&nbsp;X.)
+            macOS.)
           </p>
 
           <p>