Fix: correct return type of a method

[TheAnkurPanchani]

PR Type

Fix of a missing return type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Documentation changes
• Other... Please describe:

PR Checklist

For bug fixes / features, please check if your PR fulfills the following requirements:

• Testing has been carried out for the changes have been added
• Regression testing has been carried out for existing functionality
• Docs have been added / updated

What is the purpose of this PR?

Just fixed a return type of method in type definition file.

Does this PR introduce a breaking change?

• Yes
• No

What testing has been done on the changes in the PR?

Before change, typescript compilation was failing with following error. After change compilation was successful.

Error in cordova/plugins/cordova-plugin-firebasex/types/index.d.ts(114,5):
TS7010: 'logError', which lacks return-type annotation, implicitly has an 'any' return type.