fix(docs): Parameter names of LinearProblem in docs

The parameter names in the signature are confusing in the context of the descriptions of the parameters with the same names but different meanings under the signature in the `# Fields` section.

Author: kunzaatko

src/problems/linear_problems.jl

@@ -82,8 +82,8 @@ Optionally, an initial guess ``u₀`` can be supplied which is used for iterativ
 methods.
 
 ```julia
-LinearProblem{isinplace}(A,x,p=NullParameters();u0=nothing,kwargs...)
-LinearProblem(f::AbstractSciMLOperator,u0,p=NullParameters();u0=nothing,kwargs...)
+LinearProblem{isinplace}(A,b,p=NullParameters();u0=nothing,kwargs...)
+LinearProblem(f::AbstractSciMLOperator,b,p=NullParameters();u0=nothing,kwargs...)
 ```
 
 `isinplace` optionally sets whether the function is in-place or not, i.e. whether