Fixed all warning.warn messages for space,full-stop etc issues
I have fixed all the warning.warn messages in the entire code base for "Minor typo on warnings #39158" reported issue.

closes  #39158

Summary of the changes .
[Fix-Tracking-Sheet (2).xlsx](https://github.com/pandas-dev/pandas/files/7030053/Fix-Tracking-Sheet.2.xlsx)

Hello @Kavya9986! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:















































































There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-08-25 09:55:36 UTC Hi @Kavya9986 - few tests are failing because the old error messages are used in them. You might want to update the error msgs as well.
You can find more details of the failed tests [here](https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=64742&view=ms.vss-test-web.build-test-results-tab).
You may also run the tests locally as mentioned in the [doc](https://pandas.pydata.org/pandas-docs/stable/development/contributing_codebase.html#running-the-test-suite). > Hi @Kavya9986 - few tests are failing because the old error messages are used in them. You might want to update the error msgs as well.
> You can find more details of the failed tests [here](https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=64742&view=ms.vss-test-web.build-test-results-tab).
> You may also run the tests locally as mentioned in the [doc](https://pandas.pydata.org/pandas-docs/stable/development/contributing_codebase.html#running-the-test-suite)

Hi !
Thanks. This is very useful . I will try fixing the error messages .  Yes . I ran a script . cool, do you want to share it? perhaps a pre-commit check could be made out of it List of files changed.
[files_changed.txt](https://github.com/pandas-dev/pandas/files/7047463/files_changed.txt)

Script
```
#!/bin/bash

while IFS= read -r LINE; do
    echo $LINE
    pre-commit run flake8 --files "$LINE"
done < files_changed.txt

# files_changed.txt are the list of files that where changed since last commit

```

 I meant, to find the warnings which needed changing - I don't think `pre-commit run flake8 --files` will tell you that? Oh ! Initially I tried automating it fully , but did it partially as I thought return on my time investment would not be justified .

So first I ran this command to find the files and line number where potential fix was required 
`grep -rn 'warnings.warn(' . | cut -d":" -f1,2 | tr ":" "," > ../fix_tracking_sheet.csv`

And then filtered out all the .c files as all the warning messages there where commented .

After which I used this `grep -n8 'warnings.warn(' <File Name>` to do quick checks on each directory , fixed wherever required .

I'm still trying to use some kind of pattern-matching to automate the whole task :)

				

> I meant, to find the warnings which needed changing - I don't think `pre-commit run flake8 --files` will tell you that?

