src/sage/matrix/matrix_gf2e_dense.pyx: speed up inverse example

We have an example in this file demonstrating is_invertible() for an
element of GL(2^8, GF(2^8)). This is raising "slow doctest" warnings
on the CI and on my own machine, but the exponent is not critical to
the example. Lowering the "8" to a "6" makes the test complete much
more quickly.

Author: orlitzky

src/sage/matrix/matrix_gf2e_dense.pyx

@@ -1010,7 +1010,7 @@ cdef class Matrix_gf2e_dense(matrix_dense.Matrix_dense):
 
         EXAMPLES::
 
-            sage: m = Matrix(GL(2^8, GF(2^8)).random_element())
+            sage: m = Matrix(GL(2^6, GF(2^6)).random_element())
             sage: m.is_invertible()
             True
         """