Fixed spelling, added new words to dictionary

Change-Id: Ic1e6b69cefc362c8d512fa897188066d89824a50
diff --git a/installation.rst b/installation.rst
index 9160fb4..73177bb 100644
--- a/installation.rst
+++ b/installation.rst
@@ -249,7 +249,7 @@
 <https://github.com/opennetworkinglab/trellis-t3>`_.
 
 
-Install switch-sepcific apps
+Install switch-specific apps
 ============================
 
 There are a few switch-specific apps that need to be installed and activated additionally