docs: Add missing subcommand to manual CLI invocation example

[miguel-arrf]

Motivation

While I was doing some changes to other parts of the code, I had to manually run the CLI to obtain the generated code, however, the command was not working. Only after did I realize that the generate keyword was missing!

Modifications

Modified the Manually-invoking-the-generator-CLI.md file to fix the CLI command instruction.

Result

Users now know how to properly manually execute the CLI.

Test Plan

As of now I didn't add any tests for this. I'm not really sure where to start. My idea was to check if the function createBuildCommands from the Plugins/OpenAPIGenerator/plugin.swift file was returning the correct command with the generate argument. I was not able to import the SwiftOpenAPIGeneratorPlugin struct from a test file, meaning that I was not able to do this.

[czechboy0]

Nice catch, thank you!