Fix invalid YAML in manual.yml (quote grave accents starting scalars)

Grave accent "`" is a reserved indicator in YAML per
http://www.yaml.org/spec/1.2/spec.html#id2774228, so it can't start a
plain scalar. This commit applies quoting to titles beginning with
grave accents.

---

Note that `syck` might tolerate the invalid `manual.yml` (not sure), but `psych` won't, and upon making the following errors will be reported:

```
rake aborted!
(<unknown>): found character that cannot start any token while scanning for the next token at line 1635 column 16
.../jq/docs/Rakefile:91:in `load_manual'
.../jq/docs/Rakefile:96:in `block (2 levels) in <top (required)>'
.../jq/docs/Rakefile:95:in `block in <top (required)>'
.../.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `eval'
.../.rvm/gems/ruby-2.1.2/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => manpage
(See full trace by running task with --trace)
make[1]: *** [jq.1] Error 1
make[1]: *** Waiting for unfinished jobs....
make: *** [all] Error 2
```

where line 1635 is

```yaml
      - title: `?` operator
```