Fix cups usage by using correct request class name in CupsRequestor

[fkoehl]

1️⃣ Is this something that is wanted/needed? Did you create an issue / discussion about it first?
I need this library to direct print from my Laravel app with cups.
I did not create an issue first, should I do so? (sorry this is my first pull request).

2️⃣ Does it contain multiple, unrelated changes? Please separate the PRs out.
No.

3️⃣ Does it include tests, if possible? (Not a deal-breaker, just a nice-to-have)
No written test, but I did test with this line, and it works:
Printing::newPrintTask()->printer("ipp://localhost:631/printers/MY_PRINTER")->file('MY_FILE.pdf')->send()

4️⃣ Please include a thorough description of the improvement and reasons why it's useful.
I fixed the class name in CupsRequestor: HttpRequest instead of PendingRequest

5️⃣ Thanks for contributing! 🙌