example: fix ldb=k in example_gemm. Resolves #75.

[mgates3]

Didn't previously show because of hard-coded values n = 200 > k = 50. Using n = 50 < k = 200, it throws an error:

blaspp/examples> ./example_gemm
m 100, n 50, k 200

void test_gemm(int, int, int) [with T = float]
ldb < k, in function gemm

After this fix, it works:

blaspp/examples> ./example_gemm
m 100, n 50, k 200

void test_gemm(int, int, int) [with T = float]

void test_gemm(int, int, int) [with T = double]

void test_gemm(int, int, int) [with T = std::complex<float>]

void test_gemm(int, int, int) [with T = std::complex<double>]

void test_device_gemm(int, int, int) [with T = float]
no GPU devices

void test_device_gemm(int, int, int) [with T = double]
no GPU devices

void test_device_gemm(int, int, int) [with T = std::complex<float>]
no GPU devices

void test_device_gemm(int, int, int) [with T = std::complex<double>]
no GPU devices

[cayrols]

Looks good to me!