Add quotes around install command

Kwpolska · Kwpolska · commit 07331a665b60 · 2015-07-09T09:53:26.000+02:00
cc @obestwalter
diff --git a/docs/manual.txt b/docs/manual.txt
@@ -208,7 +208,7 @@ Note that you need Python v2.7 or newer OR v3.3 or newer.
 Some features require **extra dependencies**.  You can install them all in bulk
 by doing::
 
-    pip install nikola[extras]
+    pip install "nikola[extras]"
 
 Alternatively, you can install those packages one-by-one, when required (Nikola
 will tell you what packages are needed)
