Javascript Time Complexity: Fix Typo

[looonnng]

Because

currentNumber on line 54 is missing backticks, which made it looks inconsistent.

This PR

• Add backticks to currentNumber

Issue

Closes #XXXXX

Additional Information

Small fix so I didn't want to open an Issue, which means I couldn't give the Close# above.

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[MaoShizhong]

Just realised and checked after approving, could you apply the same fix to the Ruby version of this lesson?

[looonnng]

Done! Let me know if I could help with anything else :)