Fixed typo in Pleiades problem (docstring)

The [online docs](https://diffeq.sciml.ai/dev/types/ode_types/#DiffEqProblemLibrary.ODEProblemLibrary.prob_ode_pleiades) is incorrectly typeset due to a typo (`\\frac` instead of `\frac` inside `@doc` macro).

Author: hellmrf

src/ode/ode_simple_nonlinear_prob.jl

@@ -226,10 +226,10 @@ end
 Pleiades Problem (Non-stiff)
 
 ```math
-\\frac{d^2xᵢ}{dt^2} = \sum_{j≠i} mⱼ(xⱼ-xᵢ)/rᵢⱼ
+\frac{d^2xᵢ}{dt^2} = \sum_{j≠i} mⱼ(xⱼ-xᵢ)/rᵢⱼ
 ```
 ```math
-\\frac{d^2yᵢ}{dt^2} = \sum_{j≠i} mⱼ(yⱼ-yᵢ)/rᵢⱼ
+\frac{d^2yᵢ}{dt^2} = \sum_{j≠i} mⱼ(yⱼ-yᵢ)/rᵢⱼ
 ```
 
 where
@@ -283,19 +283,19 @@ y₆(0) = -4
 y₇(0) = 4
 ```
 
-and with ``\\frac{dxᵢ(0)}{dt}=\\frac{dyᵢ(0)}{dt}=0`` except for
+and with ``\frac{dxᵢ(0)}{dt}=\frac{dyᵢ(0)}{dt}=0`` except for
 
 ```math
-\\frac{dx₆(0)}{dt} = 1.75
+\frac{dx₆(0)}{dt} = 1.75
 ```
 ```math
-\\frac{dx₇(0)}{dt} = -1.5
+\frac{dx₇(0)}{dt} = -1.5
 ```
 ```math
-\\frac{dy₄(0)}{dt} = -1.25
+\frac{dy₄(0)}{dt} = -1.25
 ```
 ```math
-\\frac{dy₅(0)}{dt} = 1
+\frac{dy₅(0)}{dt} = 1
 ```
 
 From Hairer Norsett Wanner Solving Ordinary Differential Equations I - Nonstiff Problems Page 244