Update getting_started.md

It may be confusing to a Rails newbie why "Article" is suddenly being referred to with the "A" being capitalized. Thus far in the guide, the "a" has always been in lowercase. I think it is helpful to point out the reason for this change (especially since not using the capital "A" will result in an error). [ci skip]
Update getting_started.md

It's not very clear exactly what the "params" method is and does, as the current description in the guide is somewhat vague and brief. An example is helpful, especially considering this method is referenced over 30 times in this guide. [ci skip]
Merge pull request #15518 from jonathancadepowers/master

Added Two Clarification "Tips" [ci skip]