Rename the installed binary name

[gvangool]

When installing tfdoc from crates.io (cargo install tfdoc), it doesn't create the expected tfdoc binary. It did generate a main binary.

Based on my understanding of Cargo Targets > Binaries, we have 2 options:

• move the file from src/bin/main.rs to src/main.rs (that will make it use the package name)
• rename the file to the expected binary name (src/bin/main.rs to src/bin/tfdoc.rs)

As you can see, I opted for the second option.