Doubt in John Langford's "Tutorial on Practical Prediction Theory for Classiﬁcation" paper
I think this is a typo. 

In the Definition 2.1 and Definition 2.2, the estimation of $c_D$ is done by empirical summation of $\mathbf{1}(c(x_i) = y_i)$. 

Later in the Definition 3.1 it used the following notation : 
$$
... = \Pr_{...} \left( \sum_i Z_i &lt; k \right) = ...
$$
In this notation $Z_i=1$ corresponds to correct decisions or $c(x_i) = y_i$, and $Z_i=0$ corresponds to the mistakes or $c(x_i) \neq y_i$. I suppose that in this binomial representation $Z_i = 1$ (correct decision) corresponds to heads" (And similarly $Z_i = 0$ (wrong decision) corresponds totail"). 

That said, the right sentence seems to be "... having k or less errors is at least $\delta$"