Correct #elif typo

[Mark-Wills]

Description

File MCP7940.cpp has a typo on line 252. It should read #else, not #elif. This error prevents the library from building.

Fixes #71
In order to make tracking easier and to properly document the process,
pull requests should always refer to an active issue in the list - be it a bug fix or an enhancement or some other type of issue.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Tested with ESP32 (Arduino environment) compiler in PlatformIO.

Checklist:

• The changes made link back to an existing issue number
• I have performed a self-review of my own code
• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• The code adheres to the Google Style Guide
• The code follows the existing program documentation style using doxygen
• I have made corresponding changes to the documentation / Wiki Page(s)
• My changes generate no new warnings
• The automated TRAVIS-CI run has a status of "passed"
• I have checked potential areas where regression errors could occur and have found no issues
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes