ZEPPELIN-2985: correct grammar in keyboard tooltip

[gef756]

What is this PR for?

The tooltip for "List of shortcut" should read "List of shortcuts" (plural).

What type of PR is it?

Bug Fix

Todos

• N/A

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-2985

How should this be tested?

• Hover over tooltip. Ensure that tooltip contains changes and behaves like a
  tooltip. (Shows up then hides based on cursor position)

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[felixcheung]

ok, could you please check the rest of notebook-actionBar.html to see there is anything else to change?

[1ambda]

I tried to search other typo but couldn't find (for viable text, not for css class things). LGTM.

[Leemoonsoo]

Thanks @gef756 for the fix. LGTM

[gef756]

Cool! Do you guys need anything else from my end?

[Leemoonsoo]

Thanks @gef756 for the contribution!
Merge to master