tests/cpydiff: Fix formatting of code snippet to use double quotes.

Signed-off-by: Damien George <damien@micropython.org>

tests/cpydiff/types_float_implicit_conversion.py

@@ -2,7 +2,7 @@
 categories: Types,float
 description: uPy allows implicit conversion of objects in maths operations while CPython does not.
 cause: Unknown
-workaround: Objects should be wrapped in `float(obj)` for compatibility with CPython.
+workaround: Objects should be wrapped in ``float(obj)`` for compatibility with CPython.
 """