corrected typos in docstrings

Author: SKopecz

src/mprk.jl

@@ -824,7 +824,7 @@ These modified Patankar-Runge-Kutta methods require the special structure of a
 You can optionally choose the linear solver to be used by passing an
 algorithm from [LinearSolve.jl](https://github.com/SciML/LinearSolve.jl)
 as keyword argument `linsolve`.
-You can also choose the parameter`small_constant` which is added to all Patankar-weight denominators 
+You can also choose the parameter `small_constant` which is added to all Patankar-weight denominators 
 to avoid divisions by zero.
 
 ## References

src/sspmprk.jl

@@ -21,7 +21,7 @@ This modified Patankar-Runge-Kutta method requires the special structure of a
 You can optionally choose the linear solver to be used by passing an
 algorithm from [LinearSolve.jl](https://github.com/SciML/LinearSolve.jl)
 as keyword argument `linsolve`.
-You can also choose the parameter`small_constant` which is added to all Patankar-weight denominators 
+You can also choose the parameter `small_constant` which is added to all Patankar-weight denominators 
 to avoid divisions by zero.
 
 ## References
@@ -407,7 +407,7 @@ This modified Patankar-Runge-Kutta method requires the special structure of a
 You can optionally choose the linear solver to be used by passing an
 algorithm from [LinearSolve.jl](https://github.com/SciML/LinearSolve.jl)
 as keyword argument `linsolve`.
-You can also choose the parameter`small_constant` which is added to all Patankar-weight denominators 
+You can also choose the parameter `small_constant` which is added to all Patankar-weight denominators 
 to avoid divisions by zero.
 
 ## References