Update installation.rst

This install command does not work because `conda` must be followed by a command. See error:

```bash
CommandNotFoundError: No command 'conda conda-forge'.
```

Author: eaton-lab

docs/installation.rst

@@ -14,7 +14,7 @@ Binary packages
 The most reliable way to install ``tsinfer`` is to install the binary conda package:
 e.g.::
 
-    $ conda -c conda-forge install tsinfer
+    $ conda install tsinfer -c conda-forge
 
 you can then ``import tsinfer`` in python or use the ``tsinfer`` executable directly::