[Minor]: Incorrect word tense (introducing should be introduces)

[ChrisE2018]

Link to page(s) with issue:

• https://planning.wiki/ref/pddl3/domain

• Quote of current error:

It also introducing syntax for defining constraints.

Proposed correction:

It also introduces syntax for defining constraints.

[nergmada]

Please feel free to fork, correct and make a pull request

-Adam

[nergmada]

Pull request received and merged, thank you for taking the time to do this. If you have a chance, please see issue #95