doc: fix an error in resolution algorithm steps

As it is, if `X begins with './' or '/' or '../'` (step 3), it reads
as if it were possible for the algorithm to do a node_modules lookup
(step 4). But that doesn't seem to reflect the actual logic.

doc/api/modules.md

@@ -159,6 +159,7 @@ require(X) from module at path Y
 3. If X begins with './' or '/' or '../'
    a. LOAD_AS_FILE(Y + X)
    b. LOAD_AS_DIRECTORY(Y + X)
+   c. THROW "not found"
 4. LOAD_NODE_MODULES(X, dirname(Y))
 5. THROW "not found"