labs/kernel-serial-iomem-bbb: fix source code links

labs/kernel-serial-iomem/ moved to labs/kernel-serial-iomem-bbb/, which
didn't get reflected in the calls to the \sourcecode{} macro.

That has two side effects:
 - Labs don't the code inlined.
 - Links are invalid.

To test all links are valid, see below command (with its output before
the patch):

⟩ fd -tf -etex | \
  xargs sed -E -n 's/^.*sourcecode\{([^}]*)\}.*$/\1/p' | \
  xargs cat >/dev/null
cat: labs/kernel-serial-iomem/uarts-pinctrl.dts: No such file or directory
cat: labs/kernel-serial-iomem/uarts.dts: No such file or directory
cat: labs/kernel-serial-iomem/uart-line-init.c: No such file or directory
cat: labs/kernel-serial-iomem/uart-line-reset.c: No such file or directory

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>

Author: tleb

labs/kernel-serial-iomem-bbb/kernel-serial-iomem-bbb.tex

@@ -42,11 +42,11 @@ \section{Add UART devices}
 the standard board DTS file and create a pin muxing section with
 declarations for UART2 and UART4.
 
-{\scriptsize \sourcecode{labs/kernel-serial-iomem/uarts-pinctrl.dts}}
+{\scriptsize \sourcecode{labs/kernel-serial-iomem-bbb/uarts-pinctrl.dts}}
 
 Then, declare the corresponding devices:
 
-\sourcecode{labs/kernel-serial-iomem/uarts.dts}
+\sourcecode{labs/kernel-serial-iomem-bbb/uarts.dts}
 
 This is a good example of how we can override definitions in the Device
 Tree. \code{uart2} and \code{uart4} are already defined in
@@ -187,15 +187,15 @@ \subsection{Line and baud rate configuration}
 and configure the baud rate. This shows how to get a special
 property from the device tree, in this case \code{clock-frequency}:
 
-\sourcecode{labs/kernel-serial-iomem/uart-line-init.c}
+\sourcecode{labs/kernel-serial-iomem-bbb/uart-line-init.c}
 
 Declare \code{baud_divisor} and \code{uartclk} as \code{unsigned int}.
 
 \subsection{FIFOs reset}
 
 The last thing to do is to reset the FIFOs:
 
-\sourcecode{labs/kernel-serial-iomem/uart-line-reset.c}
+\sourcecode{labs/kernel-serial-iomem-bbb/uart-line-reset.c}
 
 We are now ready to transmit characters over the serial ports!