typo which results in bad formatting. (#3141)

removed a '`' for the markdown to parse correctly

Author: bglnelissen

userguide/install.md

@@ -20,7 +20,7 @@ source pythonenv/bin/activate
 
 To install Turi Create in the new virtual environment, do the following:
 
-```shell`
+```shell
 pip install -U turicreate
 ```