Replace 'Menlo' with 'SF Mono'

'Menlo' was replaced by 'SF Mono' from Mac OS 10.11,
And Menlo is not rendering symbols and characters correctly.

See also https://gerrit-review.googlesource.com/c/gerrit/+/246513

Change-Id: Iff8c94d5d77d7388f32bb62d67438e5e97f2ab80
diff --git a/gr-editor/codemirror-element.css b/gr-editor/codemirror-element.css
index 06c5cda..0a328b7 100644
--- a/gr-editor/codemirror-element.css
+++ b/gr-editor/codemirror-element.css
@@ -3,7 +3,7 @@
   z-index: 0;
 }
 .CodeMirror {
-  font-family: 'Roboto Mono', Menlo, 'Lucida Console', Monaco, monospace;
+  font-family: 'Roboto Mono', 'SF Mono', 'Lucida Console', Monaco, monospace;
   height: auto;
   /* CodeMirror has a default z-index of 4. Set to 0 to avoid collisions with fixed header. */
   z-index: 0;