Fixed missed alt in image::

#40

Author: nmcl

docs/manual.adoc

@@ -667,11 +667,11 @@ The maximum and minimum of the samples supplied can be obtained from the max() a
 
 sum() returns the summation of all of the samples:
 
-image::images/maths1.jpg
+image::images/maths1.jpg[]
 
 mean() returns the mean value:
 
-image::images/maths2.jpg
+image::images/maths2.jpg[]
 
 An instance of Mean can be reset between samples using the reset() method.