fix markup

Author: jepler

README.rst

@@ -58,7 +58,7 @@ To install in a virtual environment in your current project:
 CircuitPython Extensions
 ========================
 
-* `.fifo auto`: By default, CircuitPython joins the TX and RX fifos if a PIO program only receives or transmits. The `.fifo auto` directive makes this explicit.
+* ``.fifo auto``: By default, CircuitPython joins the TX and RX fifos if a PIO program only receives or transmits. The ``.fifo auto`` directive makes this explicit.
 
 Usage Example
 =============