Fix formatting in README

[aaronj0]

These extra backticks break the intended formatting. Currently we see:

## Installation within a mamba environment (wasm build instructions)

These instructions will assume you have cmake installed on your system. First clone the repository, and move into that directory
```bash
git clone --depth=1 https://github.com/compiler-research/xeus-cpp.git
cd ./xeus-cpp

[vgvassilev]

LGTM!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.94%. Comparing base (d5de374) to head (3fc6997).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #420   +/-   ##
=======================================
  Coverage   81.94%   81.94%           
=======================================
  Files          21       21           
  Lines         853      853           
  Branches       87       87           
=======================================
  Hits          699      699           
  Misses        154      154           

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