DOC: Updating grammar for clarity and consistency
### Location of the documentation
[10 minutes to pandas](https://pandas.pydata.org/pandas-docs/stable/user_guide/10min.html)

### Documentation problem
Throughout the page, there are code examples with an accompanying explanation of what is happening. The explanation will sometimes end in a colon, and other times end with a period.  Here is a screenshot of the inconsistency:

![image](https://user-images.githubusercontent.com/23269446/129495989-5b172a42-7bcc-49f4-8a4f-30d3087e872c.png)

### Suggested fix for documentation
I think that having a colon is the correct usage considering that the sentences are explaining the code block underneath it. I also think it would be better for the page to be consistent in punctuation. In my pull request, I've changed all the instances where I think this is applicable, as well as changing a couple of wordings to be easier to understand.

thanks @andrewwoood 