Manual: fix inequalities in general definition of an MCP

These should be strict inequalities.

(cherry picked from commit 3cd6bd45)

doc/manual/source/the-model-file.rst

@@ -3883,7 +3883,7 @@ speed-up on large models.
        .. math::
              LB = X   &\Rightarrow   F(X)>0\\
  
-             LB\leq X \leq UB &\Rightarrow   F(X)=0\\
+             LB < X < UB &\Rightarrow   F(X)=0\\
              X =UB &\Rightarrow   F(X)<0.