Fix unescaped backtick in documentation

On my machine(doxygen 1.16.1) the documentation task fails with:

```
include/prism/ast.h:8111: error: Reached end of file while
still searching closing '`' of a verbatim block starting at line 8080
```

Author: byroot

templates/include/prism/ast.h.erb

@@ -22,7 +22,7 @@
  */
 typedef enum pm_token_type {
 <%- tokens.each do |token| -%>
-    /** <%= token.comment %> */
+    /** <%= token.comment.gsub('`', '\\\\`') %> */
     PM_TOKEN_<%= token.name %><%= " = #{token.value}" if token.value %>,
 
 <%- end -%>