Update doc css

doc/manual-src/en/_static/default2.css

@@ -1,5 +1,5 @@
 @import url(http://fonts.googleapis.com/css?family=Roboto:400,400italic,500,500italic);
 
 pre, tt {
-    font-family: "Courier New", courier, monospace;
+    font-family:  monospace, sans-serif;
 }

doc/manual-src/ru/_static/default2.css

@@ -1,5 +1,5 @@
 @import url(http://fonts.googleapis.com/css?family=Roboto:400,400italic,500,500italic);
 
 pre, tt {
-    font-family: "Courier New", courier, monospace;
+    font-family:  monospace, sans-serif;
 }