[doc] fix markdown syntax

README.md

@@ -16,13 +16,16 @@ Gallery building needs graphicsmagick library
 
 1. Create a virtualenv, and activate it
 
-    virtualenv venv
-    source venv/bin/activate
+```
+virtualenv venv
+source venv/bin/activate
+```
 
 2. Download and install Prosopopee
 
-    pip instal git+https://github.com/Psycojoker/prosopopee
-
+```
+pip instal git+https://github.com/Psycojoker/prosopopee
+```
 
 ## Usage
 
@@ -66,4 +69,4 @@ sections:
     text: Some text, HTML <b>is allowed</b>.
   - type: bordered-picture
     image: another_picture.jpg
-```
+```