typo in manual

src/bias/External.cpp

@@ -73,11 +73,11 @@ The header in the file bias.dat for this calculation would read:
 #! SET min_t1 -pi
 #! SET max_t1 +pi
 #! SET nbins_t1 100
-#! SET peirodic_t1 true
+#! SET periodic_t1 true
 #! SET min_t2 -pi
 #! SET max_t2 +pi
 #! SET nbins_t2 100
-#! SET peirodic_t2 true
+#! SET periodic_t2 true
 \endverbatim
 
 This would be then followed by 100 blocks of data.  In the first block of data the