Fix typo in CMake options

[davidmartos96]

Description

The option FIREBASE_USE_LINUX_CXX11_ABI doesn't match the actual check the rest of the CMake project does (FIREBASE_LINUX_USE_CXX11_ABI)

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.