Fix for FS0874 error text (#9813)

Author: Lanayx

src/fsharp/FSComp.txt

@@ -716,7 +716,7 @@ tcUnnamedArgumentsDoNotFormPrefix,"The unnamed arguments do not form a prefix of
 871,tcConstructorsIllegalForThisType,"Constructors cannot be defined for this type"
 872,tcRecursiveBindingsWithMembersMustBeDirectAugmentation,"Recursive bindings that include member specifications can only occur as a direct augmentation of a type"
 873,tcOnlySimplePatternsInLetRec,"Only simple variable patterns can be bound in 'let rec' constructs"
-874,tcOnlyRecordFieldsAndSimpleLetCanBeMutable,"Only record fields and simple, non-recursive 'let' bindings may be marked mutable"
+874,tcOnlyRecordFieldsAndSimpleLetCanBeMutable,"Mutable 'let' bindings can't be recursive or defined in recursive modules or namespaces"
 875,tcMemberIsNotSufficientlyGeneric,"This member is not sufficiently generic"
 876,tcLiteralAttributeRequiresConstantValue,"A declaration may only be the [<Literal>] attribute if a constant value is also given, e.g. 'val x : int = 1'"
 877,tcValueInSignatureRequiresLiteralAttribute,"A declaration may only be given a value in a signature if the declaration has the [<Literal>] attribute"