Update become-instructor.md
Update become-instructor.md

add link to instructor training

Former-commit-id: 846ebc22b5d6a56ddcd06eb4e6fded33522a2b5f [formerly b596871450cc56a06ae7f3016ee403991518aee1]
Former-commit-id: 293b0fb92ef88b3a1f833f45565dcd9e3e6b5d62
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io