Update prepare placeholder

was missing 'install' for pip

Author: Ilya Radchenko

config/config.html

@@ -1,5 +1,5 @@
 <label for="strider-python-prepare">Prepare Command</label>
-<input id="strider-python-prepare" type="text" ng-model="config.prepare" placeholder="pip -r requirements.txt"/>
+<input id="strider-python-prepare" type="text" ng-model="config.prepare" placeholder="pip install -r requirements.txt"/>
 <p class="help-text">
   Enter which command should be used to prepare your project. The default is
   "pip -r requirements.txt". For example, this could be "python setup.py