Minor backend fr.po mistake

Hi,

#, c-format
msgid "This may be because of a non-immutable index expression."
msgstr "Ceci peut être dû à une expression d'index immutable."

Should be "NON immutable".

(Or non immuable, or is that just muable... Just kidding but I am a
bit confused about why "immutable" is preserved in English. In this
case it's referring to observed behaviour of an expression that must
have been considered immutable by static analysis but seems not to be
at execution time. In another case we use the French word "immuable"
(no T) when talking about immutability by static analysis (that is,
whether all functions are declared IMMUTABLE). Other messages and
languages made various choices about when to translate, quote or
uppercase the words immutable and volatile. Wow, this is hard.)