Typo fix in Chapter 10

Author: chrizel

10_modules.md

@@ -726,7 +726,7 @@ way to design a given module):
 
 The code used to build the road graph lives in the `graph` module.
 Because I'd rather use `dijkstrajs` from NPM than our own pathfinding
-code, we'll make this build the kind of graph data that `dijkstajs`
+code, we'll make this build the kind of graph data that `dijkstrajs`
 expects. This module exports a single function, `buildGraph`. I'd have
 `buildGraph` accept an array of two-element arrays, rather than
 strings containing hyphens, to make the module less dependent on the