Ruleset::explain(): fix plural vs singular phrasing

[jrfnl]

When running the phpcs --standard=Name -e command, the line at the top of the output would always presumes that a standard contains more than one sniff. The sniff count for the standards within the standard - StandardName (# sniff[s]) - already handled this correctly.

Fixed the top line now.

Output without this fix:

The DummySubDir standard contains 1 sniffs

DummySubDir (1 sniff)
----------------------
  DummySubDir.Demo.Demo

Output with this fix:

The DummySubDir standard contains 1 sniff

DummySubDir (1 sniff)
----------------------
  DummySubDir.Demo.Demo

[jrfnl]

@gsherwood If you're looking for another simple PR and easy merge.... 😇