[fixed] Remove required flag from contentLabel propType in Modal

[bluSCALE4]

Fixes #479.

Changes proposed:
ARIA spec, "If the label text is visible on screen, authors SHOULD use aria-labelledby and SHOULD NOT use aria-label."
Changing contentLabel propType to not be required; reference id for aria-labelledby will now be supported.

Upgrade Path (for changed or removed APIs):
Won't affect existing code.

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage remained the same at 87.24% when pulling 83ad71a on bluSCALE4:master into b908042 on reactjs:master.

[coveralls]

Coverage remained the same at 87.24% when pulling 83ad71a on bluSCALE4:master into b908042 on reactjs:master.

[diasbruno]

Hi @bluSCALE4, can you check if it is needed to update the docs?

[coveralls]

Coverage remained the same at 87.24% when pulling 7c8085c on bluSCALE4:master into b908042 on reactjs:master.

[coveralls]

Coverage remained the same at 87.24% when pulling 7c8085c on bluSCALE4:master into b908042 on reactjs:master.

[coveralls]

Coverage remained the same at 87.24% when pulling c7f79b2 on bluSCALE4:master into b908042 on reactjs:master.

[coveralls]

Coverage remained the same at 87.24% when pulling c7f79b2 on bluSCALE4:master into b908042 on reactjs:master.

[diasbruno]

Use "#".

[coveralls]

Coverage remained the same at 87.24% when pulling bc7a2c0 on bluSCALE4:master into b908042 on reactjs:master.

[coveralls]

Coverage remained the same at 87.24% when pulling bc7a2c0 on bluSCALE4:master into b908042 on reactjs:master.

[coveralls]

Coverage remained the same at 87.24% when pulling e879eff on bluSCALE4:master into b908042 on reactjs:master.

[diasbruno]

Oh, my bad. Totally forgot this PR. Sorry, @bluSCALE4. I'll get this merged soon.