add python version to conda environment creation

[tjnewton]

What does this PR do?

This PR adds a python version to the command to create a conda environment in the installation doc.

Why was it initiated? Any relevant Issues?

Default behavior initiates a conda environment with Python 3.9, which yields a conflict when installing EQcorrscan via conda with conda install -c conda-forge eqcorrscan:

UnsatisfiableError: The following specifications were found
to be incompatible with the existing python installation in your environment:

Specifications:

  - eqcorrscan -> python[version='2.7.*|3.5.*|3.6.*|>=2.7,<2.8.0a0|>=3.6,<3.7.0a0|>=3.7,<3.8.0a0|>=3.8,<3.9.0a0|>=3.5,<3.6.0a0']

Your python: python=3.9

PR Checklist

• [X] develop base branch selected?
• [X] This PR is not directly related to an existing issue (which has no PR yet).
• [ ] All tests still pass.
• [N/A] Any new features or fixed regressions are be covered via new tests.
• [N/A] Any new or changed features have are fully documented.
• [N/A] Significant changes have been added to CHANGES.md.
• [ ] First time contributors have added your name to CONTRIBUTORS.md.

[calum-chamberlain]

Golden, thanks for this - the updated docs page is here.