Update 18-environments.ipynb

fixed typo

Author: Cdribusch

book/18-environments.ipynb

@@ -1635,7 +1635,7 @@
    "source": [
     "If you use many different python environments, posssibly with different interpreter versions, you may want to learn about `pyenv` (home page at https://github.com/pyenv/pyenv).\n",
     "\n",
-    "Pyenv can install a multitude of python interpreters, create virtual environments for each of those. It is further possible to define an a *per-directory* basis which environment should be used in that directory. This is convenient when using different enviroments for different projects, as one does not need to manually activate the virtual environments."
+    "Pyenv can install a multitude of python interpreters, create virtual environments for each of those. It is further possible to define on a *per-directory* basis which environment should be used in that directory. This is convenient when using different enviroments for different projects, as one does not need to manually activate the virtual environments."
    ]
   },
   {
@@ -1680,4 +1680,4 @@
  },
  "nbformat": 4,
  "nbformat_minor": 5
-}
+}