Update README.md docs for Cronitor::Monitor.put

[nohat]

This PR updates the README.md of the Cronitor-Ruby gem, specifically revising the documentation for the Cronitor::Monitor.put method to address inaccuracies and enhance clarity.

The primary issue in the existing documentation is the incorrect usage example. It suggests that the Monitor.put method directly accepts an array of monitors, which contrasts with the actual requirement of passing these monitors within an options hash. Additionally, the current documentation lacks detail on method parameters and error handling.

This update includes:

1. A corrected usage example aligning with the method's implementation.
2. A concise overview of parameters and options.
3. A brief note on error handling associated with the method.

[jeffbyrnes]

@nohat thanks so much for this! Left one request to undo some of your changes, but the additions you’ve made are fantastic.