Renaming default branch

[awvwgk]

We should rename the default branch from master to main. See fortran-lang/fpm#421 for details.

Only two explicit references to master as default branch are in the README, so this should be straight-forward.

❯ rg master .
./README.md
27: * The API documentation for the current ```master``` branch can be found [here](https://fortran-lang.github.io/minpack/).  This is generated by processing the source files with [FORD](https://github.com/Fortran-FOSS-Programmers/ford).
31:The Minpack source code and related files and documentation are distributed under a permissive free software [license](https://github.com/fortran-lang/minpack/blob/master/LICENSE.txt) (BSD-style).