[Docu] Fix mistakes in MPL properties.

Add forgotten braces in MPL::Exponential and MPL::Linear properties.

Documentation/ProjectFile/properties/property/Exponential/c_Exponential.md

 Definition of a exponential property:
-\f$y(x) = y_{\textrm{ref}} \exp(f (x - x_{\textrm{ref}})\f$
+\f$y(x) = y_{\textrm{ref}} \exp(m (x - x_{\textrm{ref}}))\f$
 where
  - \f$y_{\textrm{ref}}\f$ is a reference value, for instance reference viscosity
  - \f$m\f$ is a factor influencing the exponent of the exponential relationship

Documentation/ProjectFile/properties/property/Linear/c_Linear.md

 Definition of a linear property:
-\f$y(x_{i}) = y_{\textrm{ref}} (1 + \sum_{i=1}^{n} m_{i} (x_{i} - x_{i,\textrm{ref}})\f$
+\f$y(x_{i}) = y_{\textrm{ref}} \left(1 + \sum_{i=1}^{n} m_{i} (x_{i} - x_{i,\textrm{ref}})\right)\f$
 where
  - \f$x_{i}\f$ can be a number of dependent variables, for instance temperature, pressure
  - \f$y_{\textrm{ref}}\f$ is a reference value, for instance reference density