fix oblique k_point definition in FAQ (fixes #2745)

doc/docs/FAQ.md

@@ -138,7 +138,7 @@ Usage: Sources
 
 ### How do I create an oblique planewave source?
 
-An arbitrary-angle planewave with wavevector $\vec{k}$ can be generated in two different ways: (1) by setting the amplitude function [`amp_func`](Python_User_Interface.md#source) to $exp(i\vec{k}\cdot\vec{r})$ for a $d-1$ dimensional source in a $d$ dimensional cell (i.e., line source in 2d, planar source in 3d), or (2) via the [eigenmode source](Python_User_Interface.md#eigenmodesource). These two approaches generate **identical** planewaves with the only difference being that the planewave produced by the eigenmode source is unidirectional. In both cases, generating an infinitely-extended planewave requires that: (1) the source span the *entire* length of the cell and (2) the Bloch-periodic boundary condition `k_point` be set to $\vec{k}$.
+An arbitrary-angle planewave with wavevector $\vec{k}$ can be generated in two different ways: (1) by setting the amplitude function [`amp_func`](Python_User_Interface.md#source) to $exp(i\vec{k}\cdot\vec{r})$ for a $d-1$ dimensional source in a $d$ dimensional cell (i.e., line source in 2d, planar source in 3d), or (2) via the [eigenmode source](Python_User_Interface.md#eigenmodesource). These two approaches generate **identical** planewaves with the only difference being that the planewave produced by the eigenmode source is unidirectional. In both cases, generating an infinitely-extended planewave requires that: (1) the source span the *entire* length of the cell and (2) the Bloch-periodic boundary condition `k_point` be set to $\vec{k}/2\pi$.
 
 The first approach involving the amplitude function is based on the principle that just as you can create a directional antenna by a [phased array](https://en.wikipedia.org/wiki/Phased_array), you can create a directional source by setting the phase of the current appropriately. Alternatively, by specifying the wavevector of the fields in $d-1$ directions of a $d$-dimensional cell, the wavevector in the remaining direction is automatically defined by the frequency $\omega$ via the dispersion relation for a planewave in homogeneous medium with index $n$: $\omega = c|\vec{k}|/n$. Note that for a pulsed source (unlike a continuous wave), each frequency component produces a planewave at a *different* angle. Also, the fields do *not* have to be complex (which would double the storage requirements).