doc: add missing line continuation character

Having missing \ characters didn't seem to break anything, but nice
to have them for consistency anyway.

Author: dlech

lib/pbio/doc/doxygen.conf

@@ -2109,9 +2109,9 @@ PREDEFINED = \
     PBIO_CONFIG_LIGHTGRID=1 \
     PBIO_CONFIG_ENABLE_SYS \
     PBDRV_CONFIG_COUNTER=1 \
-    PBDRV_CONFIG_LED=1
-    PBDRV_CONFIG_PWM=1
-    PBDRV_CONFIG_RESET=1
+    PBDRV_CONFIG_LED=1 \
+    PBDRV_CONFIG_PWM=1 \
+    PBDRV_CONFIG_RESET=1 \
     PBDRV_CONFIG_HAS_PORT_A=1 \
     PBDRV_CONFIG_HAS_PORT_B=1 \
     PBDRV_CONFIG_HAS_PORT_C=1 \