CSS fix for guides. Closing #5028 [ci skip]

In Ubuntu Chrome, in the last lines of code blocks, the underscore isn't
visible. Increasing the line height slightly seems to fix this. This
problem doesn't exist in Firefox even on Ubuntu. Too lazy to test in
any other OS-browser combo :)

railties/guides/assets/stylesheets/fixes.css

@@ -12,5 +12,5 @@
 .syntaxhighlighter table thead,
 .syntaxhighlighter table caption,
 .syntaxhighlighter textarea {
-  line-height: 1.2em !important;
+  line-height: 1.25em !important;
 }