Use math mode in two places in the manual (#1903)

Author: fingolfin

docs/src/matrix_introduction.md

@@ -1,7 +1,7 @@
 # Introduction
 
-AbstractAlgebra provides matrix spaces (mxn matrices) and matrix algebras
-(nxn matrices) over a ring. Whilst both types of matrix provide
+AbstractAlgebra provides matrix spaces ($m\times n$ matrices) and matrix algebras
+($n\times n$ matrices) over a ring. Whilst both types of matrix provide
 matrix multiplication for matrices whose dimensions are compatible for
 multiplication, only the latter kind of matrices form rings in the system.
 
@@ -16,4 +16,3 @@ field.
 
 Most of this generic functionality is provided for arbitrary matrix types
 that satisfy the AbstractAlgebra matrix interface.
-