Spell ESR as Firefox does

wiki/src/inc/release_notes/1.6.mdwn

 ## Upgrades and changes
 
   - Upgrade *Tor Browser* to version 5.0.3 (based on
-    Firefox 38.3.0esr).
+    Firefox 38.3.0 ESR).
   - Upgrade *I2P* to version 0.9.22 and enable its AppArmor profile.
 
 There are numerous other changes that might not be apparent in the daily