docs:fix typo in the README install

[yreyricord-sme]

Linked issue

Closes #47

• The linked issue is labelled status:approved, good-first-issue or help-wanted.
  Being open in the backlog is not enough on its own: only these three labels mean the scope has been triaged and approved for implementation.

What this PR changes

We correct the "runnin" typo error to running

Notes for reviewers

User-facing change

• This PR introduces a user-facing change (API, CLI, output format, performance, documentation).

If yes, write one short release-note sentence here (it will be picked up in CHANGELOG.md):

Documentation

• This PR modifies the documentation (Sphinx site, README, wiki, ATBD, Science Guide).
• This PR does not modify the documentation, and no documentation update is needed.
• This PR does not modify the documentation, but a documentation update is needed and tracked in issue #_____.

Tier rationale

The CI computes the tier automatically from the diff against the base branch. You do not assign it, but stating your expectation helps reviewers spot a mismatch quickly.

Tier	Triggers	Checks that run
0	Routine changes, no sensitive path touched	Baseline only
1	Locked paths, SME-owned paths, marker fail, Dependabot major	Baseline + Extended
2	VERSION promoted to main, designated heavy paths, manual run_heavy	Baseline + Extended + Heavy

Full rules: .github/tier-policy.yml. Background: Contribution tiers in the contributor guide.

• Expected tier:0
• Why:

AI assistance disclosure

• No AI tools were used to prepare this PR.
• AI tools were used. Tool(s):  
  
  What was generated (code, tests, documentation, commit messages):
  
  I have reviewed the generated content and take responsibility for it.

Checklist

• This PR closes exactly one tracking issue, linked above.
• The scope of the diff matches the approved scope in the linked issue. No drift, no extras.
• A breaking change is explicitly flagged in the release note sentence above (if applicable).
• The reviewer assigned has the relevant domain knowledge for this change.