Fixed type in .gitignore

[nativeit]

The .gitignore file includes entries to exclude build folders, using expressions to select for either upper-case or lower-case folder titles, but included a typo that does not exclude folders named "Build" or "build" (see diff for the specifics, but in short, a redundant "b" was erroneously included).

This PR simply corrects this typo, so the default build folder generated with CMake on Linux (and possibly others, anything that uses "./build" or "./Build") is not tracked by mistake.

Thanks to everyone who contributed to this project!