[Doc] Update the doc for Langchain::LLM::Ollama (#988)

This PR updates the doc for `Langchain::LLM::Ollama`.

In practice, `Langchain::LLM::Ollama` depends on the Faraday gem.

If `gem "faraday"` is not listed as a dependency in user's Gemfile, the following error occurs:

```console
$ bundle exec ruby example.rb
/Users/koic/.rbenv/versions/3.3.4/lib/ruby/gems/3.3.0/gems/langchainrb-0.19.5/lib/langchain/dependency_helper.rb:38:
in `rescue in depends_on': Could not load faraday. Please ensure that the faraday gem is installed. (Langchain::DependencyHelper::LoadError)
        from /Users/koic/.rbenv/versions/3.3.4/lib/ruby/gems/3.3.0/gems/langchainrb-0.19.5/lib/langchain/dependency_helper.rb:17:in `depends_on'
        from /Users/koic/.rbenv/versions/3.3.4/lib/ruby/gems/3.3.0/gems/langchainrb-0.19.5/lib/langchain/llm/ollama.rb:41:in `initialize'
```

In accordance with the langchain.gemspec, no version constraints are specified:
https://github.com/patterns-ai-core/langchainrb/blob/0.19.5/langchain.gemspec#L54

Co-authored-by: Andrei Bondarev <[email protected]>

Author: koic

lib/langchain/llm/ollama.rb

@@ -4,6 +4,9 @@ module Langchain::LLM
   # Interface to Ollama API.
   # Available models: https://ollama.ai/library
   #
+  # Gem requirements:
+  #    gem "faraday"
+  #
   # Usage:
   #    llm = Langchain::LLM::Ollama.new(url: ENV["OLLAMA_URL"], default_options: {})
   #